Imágenes de páginas

the light of Sirius and B Tauri, a star of the fecond magni(ude, is not more than as 4 10 1; while that between the for. mer and the sun is as 170,000 millions to i. The next difference between stars of the second and third inagnitude is only as 2 0 ; that between stars of the fixth and feventh magnis iude only as 1 to 1. With the naked eye, and with objects of no greater brightness than stars, we cannot probalily penetrare farther into space ; but clusters of stars, which form ne. bulæ, are seen at a still greater distance.

Our author next examines what further assistance telescopes can be expected to give. Some light must be lost by passing through the glasses. In his best telescope, not above .63 of the rays reach the eye : in a Newtonian reflector, with a double eve-glass, not many more than .40. On examination, with one of our author's twenty-feer Newtonian reflectors, made in 1776, he found that, with its affistance, he could penetrate thirty-nine times further into space than with the naked cye. In this case, the absolute not the intrinsic brightuess is increased. This leads Mr. Herschel to the distinction between magnifying and penetrating power, the latter of which only is poffeffed by the night-glasses, which penetrate fix or seven times farther than the natural eye; and the great advantages of our author's telescopes arise from their combining the penetrating and magnifying power. Various instances of the utility of oc:afionally increasing the one or other of these powers are subjoined, which can only be read with advantage in his own words. In some circuinstances, however, these powers interfere with each other; and even the magnifying power has its maximum, since, by extending it too far, obfcurity ensues from magnifying the medium. In some nights, when the air is full of vaprur, but not in the velicular state, there are scarcely any limits to the magnifying power. The penetrating power may also, in our author's opinion, be greatly extended. His forty-feet reflector advances to 191.69, but he thinks it possible to extend this power so far as 500. Even with his reflector, allowing a star of the seventh magnitude to be visible to the unaffifted eye, this telescope will ihow stars of the 1342d magnitude ; but, when affilted by the united lustre of lidercal systems, it will penetrate 11 millions of mil. lions of millions of miles, exceeding 300,000 times the distance of the nearcít fixed star! The range of such a telescope mult he of course extenlive beyond imagination, and to examine these iminense distances there are few favourable hours. Mr. Herschel, from his journal, thinks that a year, which affords go or 100 of these hours, is very productive; and to sweep the heavens' with his twenty-fcet reflector, would require 14 of such productive vears; and, with the forty-feet reActor, with the power of 1000, it will require 598 of fuele years, leaving so much of the southern hemisphere as will require 2 13 years more, allowing only one single moment to look into each part of space. i «VA second Appendix to the improved Solution of a Problem in physical Astronomy, inserted in the Philosophical Transactions for the Year 1798, containing some further Rea marks, and improved Formulæ for computing the Coefficients A and B; by which the arithmetical Work is considerably shortened and facilitated. By the Rev. John Hellins, B. D. F. R. S. and Vicar of Potter's Pury, in Northamptonshire.'

This excellent paper can only be examined with advantage in the volume.

• VI. Account of a Peculiarity in the Distribution of the Arteries sent to the Limbs of flow-moving Animals ; together with some other similar Facts. In a Letter from Mr. Anthony Carlile, Surgeon, to John Symmons, Esq. F. R. S.

The distribution of the blood-vefsels, except in the superior and inferior extremities, offers nothing very striking ; but, in these, the artery is divided at once into very many cylindrical branches, which often anastomose. The final cause of this singular arrangement is not clear. Our author thinks it is connected with the power the animal has of keeping itself, for a long time, suspended ; in other words, that it aslists the mufa cles in preserving their perinanent contraction, without alternate relaxation. It seems more probably designed to prevent obstructions, in consequence of the continued action of the muscles, or their flow motion ; for, in the more active bradypus, the B. tridactylus, the division is much less minute.

VII. Outlines of Experiments and Inquiries respecting Sound and Light. By Thomas Young, M. D. F. R. S. In a Letter to Edward Whitaker Gray, M. D. Sec. R. S.'

As the completion of Dr. Young's pursuits on this subject is yet at a distance, he has here published some of his conclus fions, leít, from accident, he may not be able to continue the inquiry. The subjects are,

II. The measurement of the quantity of air discharged through an aperture. II. The determination of the direction and velocity of a stream of air proceeding from an orifice. III. Ocular evidence of the nature of sound. IV. The velocity of sound. V. Sonorous cavities. VI. The degree of divergence of sound. VII. The decay of sound. VIII. The harmonic founds of pipes. IX. The vibrations of different elastic fluids. X. The analogy between light and found. XI. The colescence of inulical sounds. XII. The frequency of vibrations conftituting a given note. XIII, The vibrations of chords. XIV. The vibrations of rods and plates. XV. The Suman voice. XVI. The temperament of mulical intervais.' . 106.

It is impossible to follow minutely experiments reduced to the form of tables, and disquisitions, which contain a large portion of mathematical reasoning, and frequent reference to plates. We shall, therefore, only notice a few of the most striking or important passages, which do not require such assistance.

On the subject of sonorous cavities, our author confirms the observation of de la Grange, that sounds are reflected with a velocity equal to that of their impulse. When the walls of an unfurnished narrow room are parallel and smooth, found is Teflected from one to the other side, and it takes place, as frequently in a second, as double the breadth of the room is contained in 1130 feet. The appropriate notes of a room may be ascertained by singing the scale in it, and will be found to depend on the proportion of its length or breadth, to 1130 feet.

He opposes the idea of the divergence of found, with great justice. It is only surprising that this opinion has prevailed so long. Sound, he thinks, decays in the duplicate ratio of the distance, and, of course, the proposal of the improved form of the speaking trumpet, to represent the logarithmic curve, is fallacious. In the tenth section, on the analogy between light and sound, Dr. Young offers some remarks in favour of EuJer's system of light being propagated by an etherial medium.

• There are also one or two difficulties in the Newtonian system, which have been little observed. The first is, the uniform velocity with which light is supposed to be projected from all luminous bodies, in consequence of heat, or otherwise. How happens it that, whether the projecting force is the slightest transmission of electric city, the friction of two pebbles, the lowest degree of visible ignition, the white heat of a wind furnace, or the intense heat of the sun itself, these wonderful corpuscles are always propelled with one uniform velociry? For, if they differed in velocity, that difference ought to produce a different refraction. But a still inore insuperable difficulty seems to occur, in the partial reflection from every refracting surface. Why, of the same kind of rays, in every circumstance precisely similar, fome should always be reflected, and others transmitted, appears in this system to be wholly inexplicable. That a medium resembling, in many properties, that which has been denominated ether, does really exist, is undeniably proved by the phænomena of electricity; and the arguments against the existence of such an ether throughout the universe, have been pretty suffi. ciently answered by Euler. The rapid transmislion of the electrical shock Bows that the electrical medium is poflested of an elasticity as great as is necessary to be supposed for the propagation of light. Whether the electric ether is to be considered as the same with the luminous ether, if such a fluid exists, may perhaps at some future time be discovered by experiment; hitherto I have not been able to observe that the refractive power of a fluid undergoes any change by electricity. The uniformity of the motion of light in the same medium, which is a difficulty in the Newtonian theory, favours ibe adınifsion of the Huygenian; as all impressions are known to be transmitted through an elastic fluid with the famie velocity. It has been already mow'n, that found, in all probability, has very little tendency to diverge: in a medium fo highly elastic as the luminous ether must be supposed to be, the tendency to diverge may be considered as infinitely small, and the grand objection to the syslem of vibration will be removed. It is not absolutely certair, that the white line visible in all directions on the edge of a knife, in the experiments of Newton and of Mr. Jordan, was not partly occasion. ed by the tendency of light to diverge. Euler's hypothesis, of the transinillion of light by an agitation of the particles of the refracting media themselves, is liable to strong objections; according to this supposition, the refraction of the rays of light, on entering the atmosphere from the pure ether which he describes, ought to be a million times greater than it is. For explaining the phænomena of partial and total reflection, refraction, and inflection, nothing more is necessary than to suppose all refracting media to retain, by their attraction, a greater or less quantity of the luminous ether, so as to inake its density greater than that which it possesses in a vacuum, without increasing its elasticity; and that light is a propagation of an impulse communicated to this ether by luminous bodies: whether this impulse is produced by a partial emanation of the ether, or by vibrations of the particles of the body, and whether these vi. brations are, as Euler supposed, of various and irregular magnitudes, or whether they are uniform, and comparatively large, remains to be hereafter deterinined. Now, as the direction of an impulse tranf. mitted through a fluid, depends on that of the particles in fynchronous motion, to which it is always perpendicular, whatever alters the direction of the pulse, will infect the ray of light. If a smaller elastic body strike against a larger one, it is well known that the smaller is reflected more or less powerfully, according to the difference of their magnitudes : thus, there is always a reflection when the rays of light país from a rarer to a denser ftratum of ether; and frequently an echo when a sound strikes against a cloud. A greater body striking a smaller one, propels it, without losing all its motion : thus, the particles of a denser stratum of ether do not impart the whole of their motion to a rarer, but, in their effort to proceed, they are recalled by the attraction of the refracting substance with equal force; and thus a reflection is always secondarily produced, when the rays of light pass from a denser to a rarer ftratum,'

Y. 125.

" It has already been conjectured by Euler, that the colours of light consist in the different frequency of the vibrations of the luminous ether : it does not appear that he has supported this opinion

by any argument; but it is strongly confirmed, by the analogy between the colours of a thin plate and the founds of a series of ore gan-pipes. The phænomena of the colours of thin plates require, in the Newtonian system, a very complicated supposition, of an #ther, anticipating by its mocion the velocity of the corpuscles of light, and thus producing the fits of transmission and reflection; and even this supposition does not much aslift the explanation. It appears, from the accurate analysis of the phænomena which News ton has given, and which has by no means been superseded by any later 'observations, that the fame colour recurs whenever the thickness answers to the terms of an arithmetical progreflion. Now this is precisely similar to the production of the fame sound, by means of an uniform blast, froni organ-pipes which are different multiples of the same length. Suppoling white light to be a continued impulse or stream of luminous et her, it may be conceived to act on the plates as a blast of air does on the organ-pipes, and to produce vibrations regulated in frequency by the length of the lines which are terminated by the two refraéting surfaces. It may be obiected that, to complete the analogy, there should be tubes, to antwer to the organ-pipes : but the tube of an organ-pipe is only necessary to prevent the divergence of the impression, and in light there is little or no ten!ency to diverge; and indeed, in the case of a refonant paffage, the air is not prevented from becoming fonorous by the liberty of lateral motion. It would seem, that the determination of a portion of the track of a ray of light through any homoges neous stratum of ether, is sufficient to eltablish a length as a balis for colorific vibrations. In inflections, the length of the track of a ray of light through the inflecting atmosphere may determine its vibrations : but, in this case, as it is probable that there is a reflectiou from every part of the surface of the surrounding atmosphere, contributing to the appearance of the white line in every direction, in the experiments already mentioned, so it is possible that there may be fome second reflection at the immediate surface of the body itfelf, and that, by mutual reflections between these two surfaces, something like the anguiform motion suspected by Newton mav really take place; and then the analogy to the colours of shin plates will be still stronger. A inixture of vibrations, of all pollible frequencies, may casily destroy the peculiar nature of each, and concur in a general effe&t of white light.' P. 128.

On this subject we can offer no remarks, as they would lead us to considerable and disproportioned digressions. We may, however, observe, that the advantages of Euler's hypothetis, thus detailed, are partial only, and refer but to one point of the subject. The disadvantages and the discordance of this system to numerous facts, will be very obvious to the experienced philosopher ; but they appear to us to merit invc!tigation in the

« AnteriorContinuar »