number of small bones-no less than twenty-six. Even the most solid part of the foot, from the heel to the instep, that upon which the ankle rests, is in seven pieces, and there are five more in front of these before the toe bones are reached. These twelve bones form an arch upon which the whole of the body rests, and this arch is specially constructed to combine firmness with elastric spring, and in the forward or toe-part of the foot the fourteen bones, with their muscles, tendons, and ligaments, we find arrangements for considerable flexibility. None of the bones of the foot directly touch each other as bones; the junctions of all are lined with cartilaginous matter, preventing vibratory jar. A very important function, but little understood, of this complexity is to enable the foot to co-operate with the other buffers of the body in protecting the brain from concussion. In running and jumping, a mass weighing from 150 to 250 lb. is repeatedly falling to the ground, and striking it with considerable violence. If it were a rigid mass, a vibratory shock would travel through it up to the skull at each collision with the ground, and such a shock would seriously damage the brain, arresting, or more or less completely stopping, the mental functions. But we are protected against such vibratory shock-first, by the structure of the skull, which is made up of several pieces, and thus cannot vibrate as a whole; by the thick, elastic cartilages that lie between each of the twentyfour pieces of the spine; then by the cartilages of the hip and thigh-bones; by other cartilages in the kneejoints; and finally, by those of the ankle and the complex mechanism of the foot already described. As the maximum weight rests on the foot, its elastic response is primary and important. It is evident that a tight boot made of material yielding so little as thick leather, must cripple and frustrate the functions of this beautiful and beneficent mechanism. This is shown by the tottering steps of a tightly-booted human being. The tottering step of old age is largely due to the decay, the withering or drying up of and consolidating of the springs above described; but a young man or woman, or even a boy or girl, may be, in this respect, brought prematurely to a condition of old age by tight boots. This description applies not only to the torture-chambers of imbeciles who intentionally squeeze their feet in order to make them appear unnaturally small, but to ordinary average boots, such as an average boot-maker supplies when they are "made to measure.' In order to obtain a respectably fitting boot or shoe I have found it necessary to wear two or three pairs of thick woollen socks one over the other when going to be measured. A conventional fit for this enlarged foot becomes a decent fit for the actual foot. According to the natural structure of human toes, they should spread out with space between each other, forming the widest part of the foot when it is planted on the ground. They should also, by virtue of the muscles attached to them, exert some prehensile power in climbing, and in the final projecting effort of running. The victims of ordinary boots inherit degenerate feet, in which these functions are practically annulled, and before they attain middle age the large toe is actually turned inwards, and all the toes squeezed together. The naked foot of a fullgrown fine lady is usually a very disgusting object; that of an infant or a Highland lassie is very beautiful. The supply of sweat glands is area for area-more abundant on the hands than on any other part of the body, and next to the hands, the feet are the best supplied. The lesson we should learn from this is obvious enough. Both hands and feet should be specially free to perform their natural functions of superabundant exhalation. The wearing of leather gloves, merely for appearance sake, should be denounced as contemptible foppery. Protection of the hands from cold, from dirt in exceptional cases, or from mechanical injury, as in trimming thorn hedges, &c., is, of course, a full justification of the practice; but the housemaids' gloves, gardeners' gloves, driving gloves, &c., are very different from the fingerpinching hand-prisons of the fop. I have heard of women who have reduced themselves to such a condition that they are liable to take cold if their hands are wetted by water of ordinary temperature. Poor things! But what shall we say of the condition of ordinary average conventional feet? What a satire upon our ordinary practice of foot-clothing is the almost universal dread of wet feet! The human foot, with its special sole integument, is specially constructed for exposure to cold and wet ground: and, when it has not been artificially injured by false imprisonment, no inconvenience follows its free exposure to rain-sodden or snow-covered ground during the course of ordinary locomotion. Young ladies in English boarding-schools are martyrs to chilblains. Highland girls, who walk bare-footed to school through frost and snow, and rain and sleet, rarely know what such things mean, and the slight acquaintance they have is of recent origin, since their partial adoption of southern foot-gear. I have no hesitation in positively affirming that whoever is liable to take cold by temporarily wetting the feet has diseased feet. This form of disease very frequently reaches the stage of putrescence of the perspiration of the feet. We hear of many remedies for this, and of their failures, but there is one that will not fail if persistently applied, that is, continuing bare-footed and usually wet-footed. I know that it is not easy-in most cases practically impossible-to apply this fully, and caution should, of course, be used at first, but if every available opportunity were used of absolutely uncovering the feet, and at all other times wearing the most loose and porous shoes or slippers, with soft, absorbent woollen socks, great relief would be afforded. If there was no such things as tin-tacks, pins and needles, broken glass and crockery, I should be disposed to advocate the total abolition of boots, shoes and stockings, and the furnishing of all street-doors with water-troughs and towel-mats for the use of entering inmates and visitors. Natural asperities of ground (barring thorns), are easily overcome by the remarkable facility with which the sole of the foot thickens when exposed to mechanical irritation. It is a fact not generally observed that this hard integumentary structure extends not only over the bottom of the foot, but to a considerable extent up its sides, especially at the heel. I regard corns as plaintive protests made by tortured feet against the ill-usage they receive. A corn is a local demonstration of the natural thickening capabilities of the skin of the foot on the application of suitable stimulants. If the whole of the working surface were properly exposed, such callosity would extend throughout, and would be protective instead of painful. The corn is a thorn in the flesh, a local spear-like hardening, that penetrates the surrounding tenderness instead of covering and protecting it. My first experience in barefoot walking dates as far back as 1841. In the autumn of that year I was at the Athol Gathering, then a genuine Highland festival of the peasantry (subsequently a fashionable aristocratic assembly). On the morning after the ball at the Brig of Tilt, where I was the only trousered Sassenach, I started to walk to Braemar through Glen Tilt, and was overtaken by a Highland lassie, a beautiful specimen of a noble race. She had been the belle of the ball, and was engaged to its hero, a stalwart Cameron, who had carried off all the best prizes at the athletic competition. We walked on together, but though an able pedestrian, I had a struggle to keep pace with my vigorous barefooted companion. Observing this, she suggested that I should cast off my shoon and be free. I did so, and the ground being favourable for a beginner, perceived at once that my natural pace became the same as hers without the forcing or strain that was previously necessary. In the course of many thousands of miles of subsequent pedestrian excursions I have frequently done the like with similar advantage, though unable to continue a long distance on account of the tenderness of artificially-swaddled soles. Crossing a sandy bay at low tide is very tempting, and if the sands are hard a curious result follows a long walk. After a few miles a slight tenderness is felt in the ball of the large toe. This increases, and presently becomes painful. On examination, the cuticle of that part is found to be ground smoothly away by the friction on the sand. My first experience of this was in crossing one of the fine bays near the Giant's Causeway. Unconscious of what was going on I persevered until the pain grew serious, and then found that the cutis was bared. The next day I was unable to walk at all. I name this as a warning to others who may be induced to try experiments in barefoot walking. When bare feet are objectionably conspicuous, or the ground severe, I walk on for an hour or two in orthodox fashion, then stand ankle deep in a brook; or pour water in my shoes, and keep my feet scrupulously wet during the rest of the day. This adds about 20 per cent. to the possible mileage. One consequence of this, and the habit of usually wearing very loose slippers, is that, though no longer a boy, I laugh at the idea of suffering any evil consequences from wet feet, whether walking, standing, or sitting. OPTICAL RECREATIONS. Br "A FELLOW OF THE ROYAL ASTRONOMICAL SOCIETY." (Continued from Page 541 of Vol. VII.) “IF F," says Sir David Brewster in his Optics, "We transmit a beam of the sun's light through a circular aperture into a dark room, and if we reflect it from any crystallised or uncrystallised body, or transmit it through a thin plate of either of them, it will be reflected and transmitted in the very same manner, and with the same intensity, whether the surface of the body is held above or below the beam, or on the right side or left, or on any other side of it, provided that in all these cases it falls upon the surface in the same manner; or, what amounts to the same thing, the beam of solar light has the same properties on all its sides; and this is true, whether it is white light or directly emitted from the sun, or whether it is red light, or light of any other colour. The same property belongs to light emitted from a candle or self-luminous body, and all such light is called common light." Now we have previously seen in the course of these Essays (Vol. V., p. 352, et alibi) that light consists of a series of undulations or vibrations transverse to the direction in which it is propagated, and it is pretty evident that these vibrations must occur symmetrically as regards the axis of the beam. Let us suppose that enable the other to be inserted through it at right angles, and we shall obtain the V of the same figure, which will represent our symmetrical beam of lightsymmetrical because it is obviously alike in every direction. Is it possible in any way to alter this disposition of the vibrations of the ether, and so obtain a beam with sides? Let us see. Many of our London readers must recollect the shop of the late Professor Tennant in the Strand, and the splendid rhomb of Iceland spar which used to be exhibited in the window, showing a double image of a red wafer placed behind it. A description of this will form an introduction to the study of the so-called "polarisation of light," to which we are now about to address ourselves. A rhomb, as most people are aware, is a solid bounded by six equal and similar rhomboids-a rhomboid being defined by Euclid (Definition XXXIII.) as having "its opposite sides equal to each other; but all its sides are not equal, nor its angles right angles." Well, whether we find Iceland spar in a crystalline or massive condition it will always split up into rhombs, like Fig. 46. KNOWLEDGE a is the better, and at all events the length of one of its edges must be at least an inch), and making a circular dot, W, on a sheet of paper, place one face of the crystal on it. Then, to an eye placed at E, the dot will be seen double, as W, W1, and if we turn the crystal round, the same side always touching the paper, we shall see the dot W remain apparently stationary, and the dot W1 describe seeming orbit round it. Now we have several times insisted, in these papers, on the fact that light goes and returns by the same route, and hence the reader will be prepared to expect that if instead of viewing a spot from E we there place a source of light, the beam Ee will be split on entering the crystal, and its image be seen as two to an eye placed on the other side of what was the base of our crystal in our first experiment. Moreover, when we come to investigate the bending of this double ray, we shall find that while eW follows the ordinary law of sines (vol. vi., p. 32), and lies, of course, in the plane of incidence, eW1 does nothing at all of the sort, but follows some new and remarkable law. Hence eW is called the ordinary ray, and eW1 the extraordinary ray. It may not, perhaps, be wholly out of place to add here that when the ray Ee is incident perpendicularly on the face A B C D, as in other cases, the ordinary ray passes straight through the crystal, and is not refracted at all, while the extraordinary ray, under these circumstances, has an angle of refraction of 6° 12', and is bent on one side. Lastly, we may note that a ray of light passing in the direction A X suffers no double refraction whatever. 197 letting a beam of parallel solar rays r o pass through the that only the ordinary ray r o will emerge through the left hand opening, it will be seen, from what has preceded, opposite hole, the extraordinary ray re being refracted up to e, whence, of course, its egress is prevented by the solid brass plate. If now the two rhombs are so situated that their principal planes passing through ro and the optic axis lie in the same plane (say that of the paper), the ordinarily refracted ray ro will pass similarly through the second crystal, as at r'o'. If, though, keeping the first tube fixed, we rotate the second one about its axis, we shall find that double refraction will take place, and that an extraordinary as well as refracted spot of light will become visible. This extraan ordinarily ordinarily refracted ray, at first dim, becomes brighter correspondingly in illumination until the angle between as the crystal is rotated, the ordinary ray diminishing the two principal planes of cleavage of the rhombs 45°. As the rotation continues, the ordinary ray continues to fade out and the extraordinary ray to become stronger until, when the principal planes are square to each other, the ordinary ray vanishes, and the extraordinary one remains of the full strength of the polarised by the aid of slices of the mineral tourmaline, original beam o r. Light may also conveniently be cut parallel to the optic axis of the crystal. Tourmaline ray, but transmits the extraordinary ray (which vibrates of moderate thickness entirely extinguishes the ordinary parallel to its crystalline axis) perfectly. Tourmaline is of all sorts of colours, some splendid specimens from Devonshire being of so dark a brown as to appear black. If slices of sufficient thinness can be cut from almost A Fig. 47. Now we have gone into all this seemingly irrelevant detail, because in splitting the incident beam of light into two our crystal has done something else: in point of fact it has separated our original beam ABCD (Fig. 44) into two, A B and CD, each having different properties on different sides; or rather, A B has the same properties at its sides A and B that CD has at its sides C and D. In other words, the vibrations of which the beam of common light ABCD is made up have been split into two separate sets AB and CD at right angles to each other. Each, then, of these separate beams is said to be polarised, and the planes passing through the lines A B and CD are called the planes of polarisation of the beams respectively. From this we may infer, what we shall find experimentally to be the fact, that a beam of common light ABCD may be regarded as made up of two beams of polarised light with their planes of polarisation at right angles to each other; and that if we superposed these polarised beams so that their planes of polarisation, instead of being at right angles, were made coincident, we should get a beam of polarised light twice as luminous as either of the separate beams composing it. We have only to employ a second rhomb of Iceland spar to show this. For this purpose it is better to mount our two rhombs of spar, as shown in Fig. 47, where T and T' show two pieces of brass tube in which our rhombs are fastened by means of cork rings. Both ends of the tubes are covered by brass discs d d d' d', perforated centrally with holes. Placing our tubes, then, in a horizontal position, and 9 h d Fig. 48. these dark crystals, they are generally effective, but this is no easy matter. Those usually found in commerce are brown, green or red. They, of course, colour the light to a certain extent if they are of a dark tint or cut into thick slices, but this is no great practical disadvantage. Two such plates mounted in cells which can be rotated, the cells being borne at the ends of a twisted wire, are sold by opticians under the name of "Tourmaline tongs.' Any object required to be viewed by polarised light may be nipped between the wire rings in which the cells rotate. If now we place our two plates in such a position that their axes are parallel, as at A (Fig. 48). The light from the sun or any other source will pass through them just as it would through a single plate of the thickness of the two combined. If, though, we rotate one of the plates, as at B, the transmitted light will get weaker and weaker until, at length, when their axes are at right angles, it will disappear wholly, and darkness will supervene. And this seems a favourable point at which to pause in our description of Polarising apparatus, and to utilise the experiment just described in an endeavour to make the theory of polarisation apprehensible. To this end we may regard the internal structure of one of our plates as being that of a grating, with its parallel openings vertical. If now we imagine our model of a beam of ordinary light, V2, Fig. 45, to be pushed ("end on," as sailors say) against such a grating it is abundantly evident that, while the vertical series of vibrations, A B, would pass through perfectly, the transverse ones, CD, would be most effectually stopped: just as a walking-stick, held parallel to some iron railings, could be passed easily between them, but would be arrested at once if held across the direction of their length. At A, in Fig. 48, the, so to speak, cross vibrations C D, Figs. 44 and 45, have been shut out by the first plate, abcd, and only the vertical ones, AB (same figs.) allowed to pass; and as our hypothetical " grating" is also vertical in the second plate, efgh, the vertical undulations will get through that perfectly. But (B, Fig. 48) as the vibrations which pass through a b c d are all vertical, and vertical only, when we make our second supposititious grating, e f g h, horizontal, of course we interpose an insuperable barrier to their passage, and darkness is the result. (To be continued.) THE YOUNG ELECTRICIAN. BY W. SLINGO. (Continued from p. 158.) ELECTROSCOPES. R. 6.-Electroscopes are instruments for detecting the presence of a charge of static electricity, and for determining its quality. They are of a simple nature, and exhibit important and interesting phenomena, but they must not be confounded with electrometers or instruments for measuring the strength or quantity of a charge. Induction is the ruling principle in the construction and action of electroscopes. Ex. CIV. Anything which will indicate the presence of electricity (such as a suspended feather, pith-ball, &c.) is in a measure an electroscope; but we will confine ourselves to such instruments as respond to the more complete definition. Fig. 58 illustrates a typical form of instrument. It consists of a bottomless gas-receiver or glass-jar, F, in the neck of which is a hard-wood stopper or cover, D, cemented on with sealing-wax, &c., and through a hole in the centre of which is a glass or other insulating tube, C. Through C passes a brass rod, B, secured to a brass plate (or ball), A, at the top, the lower extremity being flattened out or attached to a flat crosspiece of metal, E. On to the sides of E gold-leaves or pieces of Dutch metal, LL, are gummed, or otherwise attached. Ex. CV.-A somewhat more elaborate instrument is shown in Fig. 59, in which A is the plate, B the insulated rod, and LL the leaves. In addition a metal shield, G, is fitted to the upper portion of the glass jar. It is insulated from the rod and plate A, but is connected to the earth or some other conductor in connection with it. Two strips of tinfoil, K K, are gummed or pasted on the inner surface of the jar, reaching at least high enough for the leaves, L L, to touch them instead of the glass (in the event of their being so far separated). These strips are also connected to earth. A small glass tray, H, of any convenient shape or dimensions is laid on the wooden base, W, and is supplied with a few small pieces of pumice-stone saturated with sulphuric acid, the function of which is to absorb any atmospheric moisture that may be present. Ex. CVI. A form frequently recommended for amateurs or beginners is that illustrated in Fig. 60, where F is a glass flask, which has been thoroughly dried and warmed, and in the neck of which is a sound cork or vulcanite stopper, C, through which is passed a wire, W, the upper part of which is soldered to a small disc, T, the lower part, W', being bent at right angles to carry the leaves, LL. Ex. CVII.-For our present purpose I have not a great opinion of either of these pieces of apparatus. They are one and all more costly or more elaborate than is necessary. The object of the glass jar or flask is simply to protect the leaves from air-currents, and prevent their being affected by atmospheric moisture. If we could get a transparent metallic jar, nothing could suit our purpose better, but that, of course, is out of the question. Let us study Fig. 58 and its teachings a little, and we shall then see what is actually required. Suppose an electrified rod, say of glass, to be brought near the plate A. Induction takes place, A becoming negatively, and the leaves L L positively, charged. LL, in conse quence of their similar charges, mutually repel, and, instead of hanging vertically, take up positions more or less resembling those illustrated, the amount of divergence Fig. 58. Fig. 59. Fig. 60. in any particular instrument depending naturally upon the charge. On the electrified rod being withdrawn, the leaves fall together, exhibiting no signs of electrification, the equal charges produced at A and L L combining and producing neutrality again. Let the rod be once more brought towards A, and the electroscope re-charged; then (the position of the rod being unaltered) let the finger touch B, or any part of A not immediately opposite the rod. The leaves will fall together consequent on the positive electricity repelled to them being neutralised by the earth-contact through the finger. As a matter of fact, the state of affairs is somewhat akin to a huge conductor, of which one extremity is at A and the other at some antipodean point. A Remove the finger. No change takes place in the position of the leaves. Subsequently remove the glass rod and the leaves diverge: this time with negative electricity, because the removal of the rod "sets free" the negative charge concentrated at A, some of which charge finding its way to the leaves causes the divergence. finger placed on A neutralises this negative charge. Suppose the finger not to be so placed, then we have a charge of electricity in the leaves whose quality (positive or negative) we know, because we first used a body provided with a known charge. To determine the quality of another charge all that is necessary is to place the body carrying the charge near A, and inducing a further charge in the metallic system A L. If the unknown charge is negative, a positive charge collects at A, and additional negative is driven into the leaves, whence the previously existing divergence is increased. An increased divergence then proves that the unknown charge is similar to the known charge in the leaves. The falling together (or collapse) of the leaves may indicate an opposite (in this case positive) charge on the body being tested, but there is a little chance of our being deceived in this case, as the approach of a neutral body might produce a like effect, for the charge in the leaves would speed to the assistance of the similar charge on A in its efforts to electrify the previously unelectrified or neutral rod. This may be proved by bringing the hand near A. With a ball or plate at A (instead of a metallic point, or the end of the wire) the charge on the leaves may be retained for a considerable time, but this advantage is lost if there are any points on the metallic system, or if the air confined in the jar has not been thoroughly dried. The necessity for dry warm air is apparent. There is some danger that, in the event of the charge on the rod under examination being exceptionally strong, the leaves will be made to take up positions so far divergent that they enter into contact with the surface of the glass. Now, the presence of a charge at L L indicates the ability to induce a charge on the surface of F. This certainly takes place when mutual attraction between this charge and that on LL sets in. If the surface of F is but a poor conductor, the leaves are liable to stick. This is overcome by gumming or pasting two strips of tinfoil on the inside of the glass, as K K (Fig. 59), and placing them in connection with the earth. The charge upon LL readily produces an opposite charge upon K K, the similar charge pushing its way into the earth. The divergence of L L is accordingly increased, and possibly L L comes into contact with K K, in which case neutralisation immediately ensues, and the leaves fall together uninjured. The metallic shield, G (Fig. 59), is to prevent the electrified rod exerting any direct inductive effect upon L L. The function of the sulphuric acid, with which the pumice-stone in H is saturated, is to absorb any atmospheric vapour that may be present. The objections to the form depicted in Fig. 60 are apparent. In the first place, no tin-foil strips can be utilised, and, in the second place, it is impossible to remove any atmospheric moisture, dirt, &c., which may have been left behind in sealing up. We will next turn our attention to an instrument the cost of the materials for which shall not exceed threepence, but which, nevertheless, will answer our every purpose. Therefore we have in the sun's case a much greater difficulty than in the case of Jupiter or Saturn. 66 It is true that the intense heat pervading the whole frame of the sun suggests a way of meeting the difficulty which does not at first sight seem available in dealing with the giant planets. The laws which connect density and pressure at ordinary temperatures and at ordinary pressures may probably fail altogether where the temperatures are so high and the pressures so enormous as they must be throughout the whole frame of the sun. We may say, indeed, as I have elsewhere shown, respecting the outer parts of the sun we see, what Professor Young said of the usually unseen corona, that if the term atmosphere be understood as we understand it when speaking of our own air, the gaseous regions forming the parts of the sun next within the photosphere do not form an atmosphere at all. Here are his remarks in regard to the corona, each one of them being fully applicable to the gaseous envelopes within the visible surface of the sun: Granting for the moment that the corona is in part and largely composed of an envelope of exceedingly rare gaseous matter around the sun,-then we may call it an atmosphere, because being gaseous and attached to a cosmical body, it bears to that body a relation analogous to that borne by our atmosphere to the earth itself. So far the term is a proper one. But now further, and on the contrary, the term atmosphere' carries with it to most persons certain ideas as to the distribution of temperature, density, &c., in its different parts, which are based on the fact that our terrestrial atmosphere is nearly quiescent and in static equilibrium under the force of gravity, with a temperature not more than two or three hundred degrees above the absolute zero, while the density of the portion accessible to human observation is very considerable. On the sun the conditions are immensely, and almost inconceivably different, so that the term atmosphere' becomes a very misleading one. There the equilibrium, so far as there is any, is dynamical, not statical, and the density, temperature, and condition of the gaseous substance is far more nearly that of the residual gas in a Crookes's vacuum tube through which an induction coil is sending electrical discharges; so different from that of ordinary air that Crookes thought he had found a fourth state of matter, bearing some such relation to the gaseous state as the gaseous does to the liquid." That this is so in regard to the sun is shown at once if we remember that the great openings we call spots disclose solar regions lying certainly not less than 10,000 miles below the sun's visible surface. Now the strength and breadth of the hydrogen lines seen in the spectrum of the sun's coloured flames show that the THE GREAT RED SPOT ON JUPITER. hydrogen present there is not indefinitely rarer than LE BY RICHARD A. PROCTOR. (Continued from p. 157.) ET us look into this matter a little more closely : and first, let us ask if anything akin to the difficulty thus recognised in the case of Jupiter (and also in that of Saturn) exists elsewhere. Now in the case of the sun we have an orb which is probably in large part gaseous. We certainly have, visibly, a gaseous region thousands of miles in depth, even estimating the depth only from the visible surface of luminous cloud which we call the photosphere. And in the sun's case the attraction of gravity on the atmospheric region thus recognised, is ten or twelve times greater than the attraction on the atmosphere of Jupiter. hydrogen at the pressure of the.air we breathe. Putting the pressure at the sun's visible surface at the millionth part of the atmospheric pressure on earth at the sea-level, and noting that gravity at the sun's visible surface is 27 times gravity at the earth's, we find that at a depth of two or three miles below the sun's apparent surface atmospheric pressure would be the same as at our sea-level were the same gases present, and temperature the same there as here. For in about the eighth of a mile the pressure would double, so that in 2 miles there would be twenty doublings of pressure, raising the density from the millionth part of our air's to somewhat more than equality with the density of our air (2 doubled, that double doubled, and so on, to 20 doublings, giving 1,048,576). In the next 2 miles the pressure would be increased more than a millionfold, always assuming |