mens of the fibre, and of articles produced from it, for the collections in the Museums of Economic Botany at Kew. It was with the utmost difficulty that specimens were obtained; but ample material for arriving at a definite conclusion with regard to the origin and character of "Genista fibre" was at last received. There is now in the Kew Museums a complete set, consisting of twigs, fibre in various stages of preparation, as well as yarns and coarse cloths. These were sent by Mr. Consul Perceval. There is also a sample of coarse sheeting received from M. Geoffroy St. Hilaire through the English Embassy at Paris. These fully illustrate the fibre industry connected with Spartium (Genista) junceum. "It is evident," says the Bulletin, "that this interesting rural industry is fast dying out in France. It may be said to exist now only in very remote hamlets in the Cevennes. The inquiries made by Kew were therefore only just in time to secure the last specimens of cloth made in the laborious fashion before the days of rapid communication and the introduction of cheap cotton and other goods."

ONE of the most interesting of recent additions to the Museums of Economic Botany at Kew has recently been received from Sir John Kirk. It consists of a large sheet of bark cloth prepared by the natives of Uganda from the inner bark of a species of Brachystegia, a small genus of trees belonging to the Caesalpineæ sub order of the natural order Legu. minosa. Various details relating to the use of Brachystegia as a source of bark cloth are given in the current number of the Kew Bulletin. The same number contains sections on oil palm fibre and the sources of rubber supply.

ANYONE who may desire to devote himself to the study of Finnish archæology and folk-lore will find ample material for study in the information collected by the late Dr. H. A. Reinholm. He was chaplain of the prison, and pastor of the Lutheran congregation, at the Fortress of Sveaborg, near Helsingfors, and for many years devoted the whole of his leisure time to the amassing and arrangement of facts relating to the life of the Finnish people in pist times. He died in 1883. Only a few results of his researches have been published. By far the greater part of his work is preserved in manuscript in the Historical Museum at Helsingfors. An interesting account of the labours of this indefatigable investigator is given in the current number of Globus.

MR. R. H. SCOTT delivered a lecture at the Royal United Institution on March 18, on a subject of much importance to meteorologists in this country, viz. Atlantic Weather and its connection with British Weather." He pointed out that less than a quarter of a century ago, before synchronous charts were in vogue, it would have been impossible to have traced a storm across America and the Atlantic to our own coasts; but this can now be done with considerable certainty. The broad principles which govern the weather system of the Atlantic were shown on two diagrams exhibiting the mean pressure, and the regions of greatest disturbance of temperature, on the globe, in our winter. The latter chart showed that, at that season, the relatively warmest district is near Iceland; and the barometer chart showed that close to the same region the barometer is lowest. The reasons of these relations, which involve the first principles of modern weather knowledge, were fully explained. The more northern part of the Atlantic area interests us the most. The

Of

twelve of these months have been carefully examined, and show no less than 264 depressions in various parts of the ocean. these, out of 62 which originated south of 40° N., only 16 had sufficient energy in them to cross the meridian of Greenwich, while out of 22 which originated further south only II crossed the Atlantic, and these were not all felt as actual storms in this country. The practical outcome of obtaining telegrams from America has not been satisfactory, but this failure has probably been mainly due to the fact that the reports "have been neither numerous nor full enough." This accurately represents the case at the present time; but we hope it is not too much to expect that, with our present knowledge of the paths taken by depressions with regard to areas of high pressure, some further advance may shortly be made in predicting storms by means of more numerous and fuller telegraphic reports both from outward and homeward bound ships.

66

THE following are among the lecture arrangements at the Royal Institution for the period after Easter :-Prof. T. G. Bonney, two lectures on "The Sculpturing of Britain-its Later Stages" (the Tyndall Lectures); Mr. Frederick E. Ives, two lectures on Photography in the Colours of Nature"; Prof. Dewar, four lectures on "The Chemistry of Gases"; Prof. H. Marshall Ward, three lectures on "Some Modern Discoveries in Agricultural and Forest Botany" (illustrated by lantern). The Friday evening meetings will be resumed on April 29, when a discourse will be given by Dr. William Huggins on "The New Star in Auriga"; succeeding discourses will probably be given by Captain Abney, Dr. B. W. Richardson, Mr. J. Wilson Swan, Sir James Crichton Browne, Mr. Ludwig Mond, Prof. Dewar, and other gentlemen.

A GOOD seam of coal from 7 feet to 8 feet thick has been discovered by Mr. Hughes, of the Indian Geological Survey, on the banks of the Tenasserim River, which is navigable to that point. The Government of India has sanctioned the grant of a large concession in Mergui to Ah Kwi, a wealthy Chinese resident of the Straits, to prospect for tin. According to the Calcutta correspondent of the Times, this is the first attempt to encourage on a large scale the mining industry in Mergui.

MESSRS. CROSBY LOCKWOOD AND CO. have issued the fourth edition of Mr. Primrose McConnell's "Note-book of Agricultural Facts and Figures for Farmers and Farm Students." The author was originally induced to prepare the volume by noticing the great value of Molesworth's "Pocketbook of Engineering Formula" to engineers, and of similar books to those engaged in other professions. It occurred to him that a book compiled in the same style, and devoted to farming matters, could not fail to be useful as a ready means of reference for refreshing the memory. The success of the "Notebook" has proved that he was right. The progress of agricultural practice and science has been so rapid that it has been necessary for him to rewrite the greater part of the book, and nearly twice as much matter is given in the present edition as was contained in the earlier issues. The use of a slightly longer page and thinner paper has prevented the size of the volume from being much increased.

counties of Scotland has issued its annual report for 1891. It THE Agricultural Research Association for the north-eastern includes a valuable paper on "Root Hairs," in which Mr. T.

depressions, and in order to throw some light on the origin and history of these depressions, and of the storms which they at times bring with them, various institutions have published daily maps of the weather in the Atlantic. The most complete of these maps were published by the Meteorological Office for thirteen months, commencing with August 1882. The last

carried on during the past three years. He also gives some hints on permanent pasture, and brings together various items of information which are likely to be of immediate benefit to farmers.

IF we may judge from its twenty-sixth annual report, the American Society for the Prevention of Cruelty to Animals is

doing much good work. During the past year it prosecuted no fewer than 17,847 cases in the courts. Through its efforts 49,118 disabled animals were temporarily suspended from work; 34, 264 horses, disabled past recovery, were humanely destroyed; 6444 disabled horses were removed from the streets in an ambulance. The number of prosecutions and other official interferences was larger than in previous years, but it does not follow that cruelty is more common than it was. The increase is due to the greater vigilance of the Society's officers.

In a paper contributed to the current number of the Journal of the Franklin Institute, Prof. Lewis M. Haupt argues strongly in favour of the construction of a ship canal between New York and Philadelphia, connecting the Delaware and Raritan Rivers. Such a canal would, he maintained, extend the Erie Canal and its benefits to Philadelphia, and open to its manufacturers over 16,000 miles of waterways in the great basin of the Mississippi. It would reduce the distance by water to New York harbour from 240 to about 60 miles, would afford an inside and safe passage to Eastern, Sound, and Hudson River ports; would develop a large population along the entire route, and so benefit the railroads traversing the district. "In short," says Prof. Haupt, “the effect would be to reduce the rate per mile, as well as to shorten the distance between the two greatest centres of population on the American continent, or, we may say, in the world; for nowhere else on the globe is it possible by so short and inexpensive a waterway to connect such large populations and so many and valuable interests."

is to erect at 125-ton steam It will span the

THE Bethlehem Iron Company, Pennsylvania, the Chicago Exhibition a full-size model of its hammer, said to be the largest in the world. main avenue of Machinery Hall, and will rise to a height of 90 feet. At the last Paris Exhibition great attention was attracted by a similar model shown by the Creusot works, but representing only a 100-ton hammer.

BARON VON MUELLER records, in the Victorian Naturalist for February, that, while elaborating diagnoses of new Papuan plants, he was pleasantly surprised to find among the novelties an Antholoma. This genus has hitherto been supposed to be restricted to New Caledonia. The Papuan species is dedicated to Prof. van Tieghem. The denticulation of the leaves, the elongation of the setule of the anthers and the three-celled ovulary already separate A. Tieghemi from A. montanum. Among the novelties are also Oxalis (Biophytum) albiflora, Sloanea Forbesii, which approaches S. quadrivalvis in many respects, but is petaliferous, and Quintinia Macgregori is particularly remarkable.

A "TREATISE on Physical Optics," by Mr. A. B. Basset, will be issued shortly by Messrs. Deighton, Bell, and Co.

THE proper title of Mr. A. E. H. Love's work (included in our list of forthcoming scientific books last week) is "A Treatise on the Mathematical Theory of Elasticity."

MESSRS. NALDER BROS. AND CO. have issued price lists, carefully illustrated, of their electrical testing and other scientific instruments, and of their ammeters and voltometers, resistance frames, &c.

MESSRS. DULAU AND CO. have issued Part xvii. of their "Catalogue of Zoological and Paleontological Books." contains lists of works on Mollusca and Molluscoida.

It

IN Mr. George S. Carr's letter on the terms "centrifugal force" and " force of inertia" (NATURE, p. 463), in the second sentence of the second paragraph, read "in every case as the reaction to the normal component of the centripetal force" (not “centrifugal").

AT the meeting of the Belgian Academy of Sciences on March 6, Prof. Spring announced that the late Prof. Stas had left, in an almost completed condition, a long and important memoir describing the results of several further stöchiometrical investigations. It is entitled "Silver," and will forthwith be edited, presumably by Dr. Spring, and published. It may be remembered that, after the publication of Prof. Stas's classical memoir upon the preparation of absolutely pure silver and the atomic weight of that metal, doubts were thrown by Prof. Dumas on the validity of the work on the ground that the silver employed was not free from occluded atmospheric gases. Moreover, Prof. Dumas expressed doubts as to the bearing of the work upon the celebrated hypothesis of Prout, according to which the atomic weights of all the other elements are supposed to be multiples of that of hydrogen. For, if silver possessed the atomic weight attributed to it by Prof. Stas, the atomic weight of oxygen became 15'96 and not the whole number 16, and consequently Prout's hypothesis in its original form would be negatived. In order to set these doubts at rest, and to leave his work in a perfected condition, Prof. Stas has prepared a quantity of silver with such extreme precautions that he has succeeded in obtaining it entirely free from occluded gases, and from even the minutest traces of the materials of the vessels

employed. So perfect is the purity of this silver that even when heated to the temperature of the melting-point of iridium not a trace of sodium can be detected in the spectrum of the vapour. With this silver he has repeated his former determinations of the atomic weight of the metal, and it is satisfactory to learn that the final number obtained is, as Prof. Stas himself expected it would be, identical with that formerly obtained. Hence, the objection of Prof. Dumas cannot longer be entertained, and the atomic weight of oxygen would indeed appear to be 15.96 and not 16, for the numbers obtained by Prof. Stas agree so remarkably that an error of four-hundredths of a unit would apparently be out of the question. In addition to this important memoir, Prof. Stas has also left the data of a series of twelve separate determinations of the stochiometric relation of silver to potassium chloride, the materials for which were the pure silver just described, and a specimen of potassium chloride, also prepared with a care and precaution quite in keeping with the rest of the work of the great analyst. The results of these determinations are described by Prof. Spring as agreeing in a most wonderful manner, and will afford another valuable base to which the atomic weights of many other elements may be referred. Besides these two memoirs, a third is mentioned by Prof. Spring, relating to the spectra of several metals which Prof. Stas has obtained in the purest state in which these metals have ever probably been seen. The whole of these memoirs, consisting of about fifteen hundred pages of manuscript, it is intended to publish forthwith in three separate treatises.

THE additions to the Zoological Society's Gardens during the past week include a Ring-necked Parrakeet (Palæornis torquatus ¿) from India, presented by Mr. George H. Whitaker; a Greybreasted Parakeet (Bolborhynchus monachus) from Monte Video, presented by Miss Mildred Whitaker; a Roseate Cockatoo (Cacatua roseicapilla) from Australia, presented by Mr. J. S. Gibbons; a Nutmeg Fruit Pigeon (Carpophaga bicolor) from the Torres Straits, presented by Mrs. Fitzgerald; two Pike (Esox lucius) from British Fresh Waters, presented by Mr. P. F. Coggin; a Mantchurian Crane (Grus viridirostris) from North China, deposited.

OUR ASTRONOMICAL COLUMN. FUZZINESS OF SOME VARIABLE STARS.- Mr. Cuthbert G. Peek has, during the last six years, used his 61-inch achromatic for the investigation of the light-curves of variable stars. In

this month's Knowledge he describes some observations of changes in appearance of a few variable stars at different epochs. Three variables-T Cassiopeia, R Cassiopeiæ, and S Herculishave been frequently observed as (a) remarkably well defined, almost planetary, disks; (b) well-defined stars, surrounded by a more or less dense, ruddy atmosphere; (c) large, woolly, illdefined images, resembling a small but bright planetary nebula ; (d) at minimum, in place of the variable, a slight bluish nebulosity. The changes appear to be real, for stars near the places of the variables have been seen clear and sharp when the haziness of the variables was unmistakable. Other stars with regard to which Mr. Peek has made similar observations are S Cassiopeia, R Tauri, R Auriga, V Cancri, R Ursa Majoris, S Ursa Majoris, R Camelopardi, R Bootis, S Coronæ, R Aquile, and S Cephei.

ASTRONOMICAL POSSIBILITIES AT CONSIDERABLE ALTITUDES.-Prof. Pickering, in No. 3079 of the Astronomische Nachrichten, relates some interesting facts in an article on "Astronomical Possibilities at Considerable Altitudes." They are gleaned from observations made at the Boyden Station of the Harvard College Observatory, which is situated two miles from the city of Arequipa, Peru, in latitude 16° 24' S., and longitude 4h. 45m. 30s. W. of Greenwich, and at an altitude of 8060 feet above sea-level. The air there is so clear and steady that 65 magnitude stars are picked out by the naked eye with great ease, and, when the moon is not too bright, the eleven Pleiads can always be counted. The nebula in Andromeda forms also a very conspicuous object, appearing larger than the moon," while, in the 13-inch Clark refractor, "the whole photographic region of the great Orion nebula, first shown in the Harvard photographs of 1887, is clearly visible to the eye," rendering it the "most splendid object in the stellar universe." The steadiness of the atmosphere is also very much remarked there, so much so that a scale of steadiness has been adopted. Some of the brightest stars have been noticed to have as many as six complete diffraction rings round them; while around these, when the seeing was denominated as "perfect," twelve rings have been counted. "Boiling was also found to be sometimes completely eliminated, for, in observing bodies of the solar system with a 13-inch and a power of 400, it was frequently impossible to detect any wavering of the edges of the disk."

The conclusion that Prof. Pickering comes to with regard to the position of future Observatories is that "moderate altitude is a most desirable qualification," while "for transparent skies one must approach the tropics, and for steady seeing one must have an extremely dry climate."

INCREASE OF THE EARTH'S SHADOW DURING LUNAR ECLIPSES. In a memoir with the title "Die Vergrösserung des Erdschattens bei Mondfinsternissen" (Abhandlungen der math. phys. Classe der k. Sachsischen Ges. d. Wissenschaften, vol. xvii., Lepzig, 1891), Dr. Hartmann published the results of an investigation into the amount by which the earth's atmosphere increases the diameter of the section of the shadow during a lunar eclipse. An abstract of the memoir appears in the annual report of the Royal Astronomical Society, which has just been issued. Since the time of Tobias Mayer (1750) the coefficient

has been assumed to represent this increase, although nothing is known as to the manner in which this quantity was determined. Dr. Hartmann has reduced all the observations of lunar eclipses observed independently by several astronomers during this century, and has deduced the increase of the diameter of the shadow from them. The result of a comprehensive discussion of 2920 observations of the contact of the shadow with well-defined lunar formations is, that the increase of the semi-diameter of the shadow is 48" 62 for mean lunar parallax. This corresponds to a coefficient of increase The result may perhaps be 50.79' changed 2" or 3" by a discussion of new observations, but not more, so it seems desirable that the value of should be used, when required, instead of Mayer's value of

I

THE NEW STAR IN AURIGA.

THE new star is rapidly getting more and more difficult of observation in cons-quence of its waning light. There is very little change in its spectrum, and what change there is is not in the direction recorded of Nova Cygni, so it seems pretty

clear that the new body was not a hitherto unobserved nebula to begin with.

The Astronomische Nachrichten, No. 3079, contains (at p. 109) the following communication from Mr. H. C. Vogel, Director of the Astro-physical Observatory at Potsdam, dated February 29:

"Although the spectroscopic observations of the Nova in Auriga are not yet concluded-since the star will probably continue visible for some time-I consider it of importance, in the interest of the subject, to communicate my observations made hitherto, and the conclusions drawn therefrom, even though the latter should not in the future be confirmed in all points.

"Concerning, first, the direct spectroscopic observations, I have, on February 20, observed the Nova with a compound spectroscope of a dispersion sufficient just to show the nickel line between the D lines. The hydrogen lines C, F, and Hy appeared bright. Their identification was easy by means of a hydrogen tube in front of the slit. These three lines did not exactly coincide with the lines of the comparison spectrum, but were displaced considerably towards the red, without, however, separating completely from the artificial lines, since they were very broad. The continuous spectrum appeared faint, owing to

the comparatively high dispersion; and with certainty only the dark broad F line was recognizable, situate towards the more refrangible side, distinctly separated from the bright line in the spectrum.

66

'Between C and F, a large number of bright lines could be seen, but most of them were too faint to be fixed with certainty. In the case of two brighter lines near F, myself and Mr. Frost, who assisted in the observations, succeeded in making very certain wave-length determinations; we found 492.5 μu for the fainter of the two lines, which appeared broad and fuzzy on both edges, and 501 6 uu for the brighter line. The limit of error is to be taken at about 3 μu, and it results from the observation with certainty that the brighter line is not identical with the double line of the air spectrum or with the brightest line of the nebula, and still less the other with the second nebula line. From Young's list of lines most frequent in the chromosphere, it follows that near F only the two groups of lines, 501 87, 501 59, and 493 44, 492'43, 492 24, 49192 frequently appear bright. There is no doubt that both lines in the spectrum of the Nova are chromosphere lines, and this result appears to me of great importance, in so far as it is made probable that the line observed in Nova Cygni (1876)—W.L. 500 μμ ± 1 μμ-which, during the gradual fading of the star, alone remained, was a chromosphere line, and not the nebula line.

"Further, both myself and Mr. Frost saw probably the magnesium lines, certainly the sodium lines bright, as also two lines between band D, one of which probably was the well-known chromosphere line W. L. 531 72, also observed in Nova Cygni. By direct comparison with the hydrocarbon spectrum, the brightest band of which nearly coincides with the group, and with the sodium flame, b and D were identified. Mr. Frost could see a displacement of the D lines in the star spectrum with respect to the comparison spectrum. There was no indication of hydrocarbon bands in the spectrum of the Nova.

"Up to the present eleven mostly very good spectrographic photographs have been taken; they were obtained by means of a small spectrograph connected to the photographic refractor of 34 cm. aperture. The dispersion is only small, but in the small spectrum of 10 mm. length, extending from F to H, much detail is discernible. The illuminating power of the apparatus is so great, in spite of the narrow slit employed, that even now an exposure of 40 minutes is sufficient to obtain an image suitable for measurement. The bright hydrogen lines F, Hy, k, H, and the calcium line H, are very broad; and, as already announced, the corresponding dark lines of a second spectrum are displaced with respect to the bright lines towards the violet, and in spite of the breadth of the latter, are almost entirely separated. There are still some of the hydrogen lines in the ultra-violet visible, but they are too faint for any approximately certain observation.

"In the last few days the spectrum has changed, inasmuch as in the broad bright lines Hy, h, H, and H, (F is only traced on plates which are over-exposed for the middle of the photographic spectrum), two maxima of intensity are plainly discernible, and, as in each of the corresponding dark lines, a narrow bright line has appeared. From the measurements, a connection between these and the hydrogen lines appears beyond doubt, and it is

not improbable that these linear brightenings in the broad dark lines indicate eruptions of gases from the interior of the body possessing the continuous spectrum with the dark absorption lines. Such brightenings are occasionally seen in the spectra of sun-spots. On this supposition, the fine bright lines would indicate very nearly the middle of the dark lines.

"The appearance of two maxima of intensity in the broad bright lines admits of the conclusion that two bodies with different motions possess spectra with bright lines, and that therefore the spectrum of the Nova consists of at least three spectra superposed, from the measurement of which, in connection with the comparison spectra of ß Auriga or 8 Tauri on the >ame plate, the relative motions of the three supposed bodies, as well as their motions with respect to the earth, can be determined. Denoting the body with the dark-line spectrum by a, the two others with bright-line spectra by b and c, measurements by myself and Dr. Scheiner have given the following results : 120 miles,1

a

(b + c) b

=

-= 70 miles.

and further with respect to the earth

a =

90 miles, b = 5,65 miles. "This result is still very uncertain, and must be regarded as quite preliminary, for it is evident that with the small size of the spectra the accuracy cannot be pushed very far-a displacement of or mm. corresponds, for instance, to a motion of 8 to 12 miles, according to the situation of the line in the spectrumand that the size of the silver grain in the photographs can exert a very marked influence on the measurements.

"In the photographic spectrum of the Nova, besides the broad lines mentioned, several more bright and mostly very broad lines can be seen, whose wave-lengths I intend to communicate later on."

Prof. Pickering communicates some valuable information to the same number of the Astronomische Nachrichten with reference to the visibility of the Nova before its discovery by Dr. Anderson. In eighteen photographs of this region, which were taken by the 8-inch photographic telescopes between the dates November 3, 1885, and November 2, 1891, no star in the Nova's place was visible, but in those taken from December 16, 1891, to January 31, 1891, there was a star of the fifth magnitude recorded. In another series of plates taken with the transit photometer, no record of the new star up to December 1, 1891, was obtained, although x Auriga (mag. 50 m.) was always visible, but the plates taken on the nights of December 10, 1891, and ending January 20, 1892, indicated clearly the position of the new star.

Careful examination has been made on all the above-mentioned

plates, and the following extract shows the series of magnitudes

which have been deduced from the measurements:

"It appears that the star was fainter than the eleventh magnitude on November 2, 1891, than the sixth magnitude on December 1, and that it was increasing rapidly on December 10. A graphical construction indicates that it had probably attained the seventh magnitude within a day or two of December 2, and the sixth magnitude December 7. The brightness increased rapidly until December 18, attaining its maximum about December 20, when its magnitude was 4'4m. It then began to decrease slowly, with slight fluctuations, until January 20, when it was slightly below the fifth magnitude."

From this it will be seen that two months' observations have been lost, owing to its late discovery.

ABERRATION?

UNDER this head may conveniently be considered not only the

apparent displacement of the stars discovered by Bradley, but other kindred phenomena dependent upon the velocity of light bearing but a finite ratio to that of the earth in its orbit round the sun, and to other astronomical velocities.

The explanation of stellar aberration, as usually given,

1 = about 540 English miles.-Tr.

This paper was written in 1887, when I was occupied with my article upon Wave Theory" for the "Encyclopædia Britannica," and at a time when a more extensive treatment was contemplated than was afterwards found practicable. Friends on whom I can rely are of opinion that its publication may be useful; and, as I am not able to give it a complete revision. I prefer to let it stand under its original date, merely warning the reader that very important work has since been published by Michelson.-January 1892.

proceeds rather upon the basis of the corpuscular than of the wave theory. In order to adapt it to the principles of the latter theory, Fresnel found it necessary to follow Young in assuming that the æther in any vacuous space connected with the earth (and therefore practically in the atmosphere) is undisturbed by the earth's motion of 19 miles per second. Consider for simplicity the case in which the direction of the star is at right angles to that of the earth's motion, and replace the telescope, which would be used in practice, by a pair of perforated screens, on which the light falls perpendicularly. We may further imagine the luminous disturbance to consist of a single plane pulse. When this reaches the anterior screen, so much of it as coincides with the momentary position of the aperture is transmitted, and the remainder is stopped. The part transmitted proceeds upon its course through the æther independently of the motion of the screens. In order, therefore, that the pulse may be transmitted by the aperture in the posterior screen, it is evident that the line joining the centres of the apertures must not be perpendicular to the screens and to the wave front, as would have been necessary in the case of rest. For in consequence of the motion of the posterior screen in its own plane the aperture will be carried forward during the time of passage of the light. By the amount of this motion the second aperture must be drawn backwards, in order that it may be in the place required when the light reaches it. If the velocity of light be V, and that of the earth be v, the line of apertures, giving the apparent direction of the star, must be directed forwards through an angle equal to /V. More generally, if the angle between the star and the point of the heavens towards which the earth is moving be a, there will be an apparent displacement towards the latter point, expressed by sin a. v/V, and independent of the position upon the earth's surface where the observation is made. The ratio /V is about rooo.

The aperture in the anterior screen corresponds to the objectglass of the telescope with which the observation would actually be made, and which is necessary in order to produce agreement of phase of the various elementary waves at a moderately distant focal point. The introduction of a refracting medium would complicate the problem, and is not really necessary for our present purpose. As has been shown (Philosophical Magazine, March 1881, "On Images formed without Reflection or Refraction"), the only use of an object-glass is to shorten the focal length. Our imaginary screens may be as far apart as we please, and if the distance is sufficient, the definition, and consequently the accuracy of alignment, is as great as could be attained with the most perfect telescope whose aperture is equal to that in the

anterior screen.

It appears, then, that stellar aberration in itself need present no particular difficulty on the wave theory, unless the garded as such. But there are a variety of allied phenomena, hypothesis of a quiescent æther at the earth's surface be remostly of a negative kind, which require consideration before any judgment can be formed as to the degree of success with which the wave theory meets the demands made upon it. In the first place, the question arises whether terrestrial optical phenomena could remain unaffected by the supposed immense relative motion of our instruments and of the æther; whether reflection, diffraction, and refraction, as ordinarily observed by us, could be independent of the direction of the rays relatively to the earth's motion. It may be stated at once that no such influence has been detected, even in experiments carefully designed with this object in view.

Another class of experiments, with the results of which theory must be harmonized, are those of Fizeau and Michelson upon the velocity of light in ponderable refracting media which have a rapid motion (relatively to the instruments and other surrounding bodies) in the direction of propagation, or in the opposite direction. These very important researches have proved that in the case of water the velocity of the ponderable medium is not without effect; but that the increment or decrement of the velocity of propagation is very decidedly less than the velocity On the other hand, the motion of air, even at

of the water.

high velocities, has no perceptible influence upon the propagation of light through it.

Again, it has been found by Airy, as the result of an experiment originally suggested by Boscovitch, that the constant of stellar aberration is the same, whether determined by means of a telescope of the ordinary kind, or by one of which the tube is filled with water. It is clear that, according to Fresnel's views I Proc. Roy. Soc., xx., 1872, p. 35; xxi., 1873, p. 121.

of the condition of the æther at the earth's surface, this agreement must involve some particular supposition as to the propagation of light in moving refracting media.

The theory of these phenomena must evidently turn upon the question whether the æther at the earth's surface is at rest, absolutely, or relatively to the earth; and this fundamental question has not yet received a certain answer. The independence of terrestrial optical phenomena of the earth's motion in its orbit is, of course, more easily explained upon the latter alternative; or rather no explanation is required. But in that case the difficulty is thrown upon stellar aberration, which follows a more simple law than we should expect to apply in the case of an æther disturbed by the passage of a body in its neighbourhood. Prof. Stokes has, indeed, attempted a theory on these lines,2 by supposing the ætherial motion to be what is called in hydrodynamics irrotational. In strictness there is, however, no such motion possible, subject to the condition of vanishing absolutely at a great distance, and relatively at the earth's surface; and it does not appear that the objection thus arising can be satisfactorily met.

If we start from the experimental facts which have the most direct bearing upon the question under discussion, we are led to regard Fresnel's views (doubtless in some generalized form) as the more plausible. From the results of Fizeau and Michelson relative to air, we may conclude with tolerable confidence that a small mass of ponderable matter, of very low refracting power, moving in space, would not appreciably carry the æther with it. The extension of the argument to a body as large as the earth is not unnatural, though it involves certainly an element of hypothesis. In like manner, if the globe were of water, we should expect the æther to be carried forward, but not to the fall amount. The simple t supposition open to us is that, in any kind of ponderable matter, forming part of a complex mass, the æther is carried forward with a velocity dependent upon the local refracting power, but independent of the refracting power and velocity of other parts of the mass. In the earth's atmosphere, where the refracting power is negligible, the ther would be sensibly undisturbed.

=

If we agree to adopt this point of view provisionally, we have next to consider the relation between the velocity of luminous propagation in moving ponderable matter and the refractive index. The character of this relation was discovered by Fresnel, whose argument may be thrown into the following form. Consider the behaviour of the æther when a plate of ponderable matter (index μ) is carried forward through vacuum with velocity in a direction perpendicular to its plane. If D be the density of the æther in vacuum, and D, the density in the refracting medium, then, according to Fresnel's views as to the cause of refraction, D1 = D. The æther is thus condensed as the plate reaches it; and if we assume that the whole quantity of ather is invariable, this consideration leads to the law giving the velocity (az) with which the denser æther within the plate must be supposed to be carried forward. For conceive two ideal planes, one in the plate and one in the anterior vacuous region, to move forward with velocity v. The whole amount of æther between the planes must remain unchanged. Now, the quantity entering (per unit area and time) is Dv, and the quantity leaving is D1(vxv). Hence,

x = I με,

so that the velocity with which the æther in the plate is carried forward is v(I -μ2), tending to vanish as μ approaches unity. If V be the velocity of light in vacuum, and V/μ the velocity in the medium at rest, then the absolute velocity of light in the moving medium is

(1) Whatever may be thought of the means by which it is obtained, it is not a little remarkable that this formula, and no other, is consistent with the facts of terrestrial refraction, if we once admit that the æther in the atmosphere is at absolute rest. It is not probable that the æther, in moving refracting bodies, can properly be regarded as itself in motion; but if we knew more about the matter we might come to see that the objection is verbal rather than real. Perhaps the following illustration may assist the imagination. Compare the æther in vacuum to a stretched string, the transverse vibrations of which represent

1 An accusation of crudeness might fairly be brought against this phraseology; but an attempt to express the argument in more general language would probably fail, and would in any case be tedious.

2 Phil. Alag., xxviii., 1846, p. 76; xxix., 1846, p. 6.

light. If the string is loaded, the velocity of propagation of waves is diminished. This represents the passage of light through stationary refracting matter. If now the loads be imagined to run along the string with a velocity not insensible in comparison with that of waves, the velocity of the latter is modified. The substitution of a membrane for a string will allow of a still closer parallel. It appears that the suggested model would lead to a somewhat different law of velocity from that of Fresnel; but in bringing it forward the object is merely to show that we need not interpret Fresnel's language too literally.

We will now consider a few examples of the application of the law of velocity in a moving medium; and first to the experiment of Boscovitch, in which stellar aberration is observed with a telescope filled with water. We have only to suppose the space between the two screens of our former explanation to be occupied by water, which is at rest relatively to the screens. In consequence of the movement of the water, the wave, after traversing the first aperture, is carried laterally with the velocity (1-2), and this is to be subtracted from the actual velocity of the aperture in the posterior screen. The difference is μ-2v. The ratio of this to the velocity of light in water (V/u) gives the angular displacement of the second aperture nece-sary to compensate for the motion. We thus obtain μv/V. This angle, being measured in water, corresponds to /V in air; so that the result of the motion is to make the star appear as if it were in advance of its real place by the angle v/V, precisely as would have happened had the telescope contained air or vacuum instead of water.

We will now calculate the effect of the motion of a plate perpendicular to its own plane upon the retardation of luminous waves moving in the same (or in the opposite) direction. The velocity of the plate is v, its index is u, and its thickness is a. Denoting, as before, the velocity of the æther within the plate by xv, and supposing, in the first place, that the signs of and V are the same, we have, for the absolute velocity of the wave in the plate,

V/μ + xv.

If we suppose that part of the original wave traverses the plate, and that part passes alongside, (5) gives the relative retardation -that is, the distance between the wave fronts which were originally in one plane. It would appear at first sight that this result would give us the means of rendering v evident. For the retardation, depending upon the sign of a/V, will be altered when the direction of the light is reversed, and this we have it in our power to bring about by simply turning our apparatus through 180°. A more careful examination will, however, lead us to a different conclusion.

The most obvious way of examining the retardation would be to use homogeneous light, and, by producing regular interference of the two portions, to observe the position of the fringes, and any displacement that might result from a shift of the apparatus relatively to the direction of the earth's motion. But if we employ for this purpose a terrestrial flame, e.g. that of a Bunsen's burner containing sodium, we have to take into