each other through intervening molecules of the same kind, and the qualities of matter as depending on the motions of different orders of molecules. In pursuance of the same views, magnetic attractions and repulsions were to be referred to differential conditions of pressure.

Physical Considerations regarding the Possible Age of the Sun's Heat.
By Professor W. THOMSON, F.R.S.

The author prefaced his remarks by drawing attention to some principles previously established. It is a principle of irreversible action in nature, that, "although mechanical energy is indestructible, there is a universal tendency to its dissipation, which produces gradual augmentation and diffusion of heat, cessation of motion, and exhaustion of potential energy, through the material universe." The result of this would be a state of universal rest and death, if the universe were finite and left to obey existing laws. But as no limit is known to the extent of matter, science points rather to an endless progress through an endless space, of action involving the transformation of potential energy through palpable motion into heat, than to a single finite mechanism, running down like a clock and stopping for ever. It is also impossible to conceive either the beginning or the continuance of life without a creating and overruling power. The author's object was to lay before the Section an application of these general views to the discovery of probable limits to the periods of time, past and future, during which the sun can be reckoned on as a source of heat and light. The subject was divided under two heads: 1. On the secular cooling of the sun; 2. On the origin and total amount of the sun's heat.

In the first part it is shown that the sun is probably an incandescent liquid mass, radiating away heat without any appreciable compensation by the influx of meteoric matter. The rate at which heat is radiated from the sun has been measured by Herschel and Pouillet independently; and, according to their results, the author estimates that if the mean specific heat of the sun were the same as that of liquid water, his temperature would be lowered by 1°4 Centigrade annually. In considering what the sun's specific heat may actually be, the author first remarks that there are excellent reasons for believing that his substance is very much like the earth's. For the last eight or nine years, Stokes's Principles of Solar and Stellar Chemistry have been taught in the public lectures on natural philosophy in the University of Glasgow; and it has been shown as a first result, that there certainly is sodium in the sun's atmosphere. The recent application of these principles in the splendid researches of Bunsen and Kirchhoff (who made an independent discovery of Stokes's theory) has demonstrated with equal certainty that there are iron and manganese, and several of our other known metals, in the sun. The specific heat of each of these substances is less than the specific heat of water, which indeed exceeds that of every other known terrestrial solid or liquid. It might therefore at first sight seem probable that the mean specific heat of the sun's whole substance is less, and very certain that it cannot be much greater, than that of water. But thermodynamic reasons, explained in the paper, lead to a very different conclusion, and make it probable that, on account of the enormous pressure which the sun's interior bears, his specific heat is more than ten times, although not more than 10,000 times, that of liquid water. Hence it is probable that the sun cools by as much as 14° C. in some time more than 100 years, but less than 100,000 years.

As to the sun's actual temperature at the present time, it is remarked that at his surface it cannot, as we have many reasons for believing, be incomparably higher than temperatures attainable artificially at the earth's surface. Among other reasons, it may be mentioned that he radiates heat from every square foot of his surface at only about 7000 horse-power. Coal burning at the rate of a little less than a pound per two seconds would generate the same amount; and it is estimated (Rankine, Prime Movers,' p. 285, edit. 1859) that in the furnaces of locomotive engines, coal burns at from 1 lb. in 30 seconds to 1 lb. in 90 seconds per square foot of grate-bars. Hence heat is radiated from the sun at a rate not more than from fifteen to forty-five times as high as that at which heat is generated on the gratebars of a locomotive furnace, per equal areas.

The interior temperature of the sun is probably far higher than that at the surface, because conduction can play no sensible part in the transference of heat between

the inner and outer portions of his mass, and there must be an approximate concective equilibrium of heat throughout the whole; that is to say, the temperatures at different distances from the centre must be approximately those which any portion of the substance, if carried from the centre to the surface, would acquire by expan sion without loss or gain of heat.

PART II. On the Origin and Total Amount of the Sun's Heat.

The sun being, for reasons referred to above, assumed to be an incandescent liquid now losing heat, the question naturally occurs, how did this heat originate? It is certain that it cannot have existed in the sun through an infinity of past time, because as long as it has so existed it must have been suffering dissipation; and the finiteness of the sun precludes the supposition of an infinite primitive store of heat in his body. The sun must therefore either have been created an active source of heat at some time of not immeasurable antiquity by an overruling decree; or the heat which he has already radiated away, and that which he still possesses, must have been acquired by some natural process following permanently established laws. Without pronouncing the former supposition to be essentially incredible, the author assumes that it may be safely said to be in the highest degree improbable, if, as he believes to be the case, we can show the latter to be not contradictory to known physical laws.

The author then reviews the meteoric theory of solar heat, and shows that, in the form in which it was advocated by Helmholz*, it is adequate, and it is the only theory consistent with natural laws which is adequate, to account for the present condition of the sun, and for radiation continued at a very slowly decreasing rate during many millions of years past and future. But neither this nor any other natu ral theory can account for solar radiation continuing at anything like the present rate for many hundred millions of years. The paper concludes as follows:-"It seems therefore, on the whole, most probable that the sun has not illuminated the earth for 100,000,000 years, and almost certain that he has not done so for 500,000,000 years. As for the future, we may say with equal certainty that inhabitants of the earth cannot continue to enjoy the light and heat essential to their life for many million years longer, unless new sources, now unknown to us, are prepared in the great storehouse of Creation."

LIGHT, HEAT.

On Photographic Micrometers. By Sir DAVID BREWSTER, K.H., F.R.S. When examining, several years ago, some microscopic photographs executed by Mr. Dancer, the celebrated optician of this city, the author was struck with the singular sharpness and opacity of some of the lines in such of them as were copied from engravings. The idea occurred to him of obtaining photographically, by means of the camera, micrometrical scales, or systems of delicate lines, opake or transpa rent, and fitted both for astronomical and microscopical purposes. The suggestion was published in the article "Micrometer" in the Encyclopædia Britannica. Mr. Dancer had succeeded in making photographic portraits on collodion so small that they were wholly invisible to the naked eye, and 10,000 portraits might be introduced into a square inch. The film of collodion upon which these photographs were taken was so thin and transparent that it was invisible, and allowed objects to be seen through it as distinctly as if it were the thinnest glass. If a system of opake or transparent lines, therefore, was impressed upon collodion or albumen photographically, when reduced to the minutest size from a system of large and sharply-defined lines, we should have the most perfect micrometrical scale that could be conceived. In the Philosophical Magazine' for August 1861, Dr. Woods, of Parsonstown, had suggested the construction of photographic micrometers without being aware of what had been published on the subject.

*Popular Lecture delivered at Königsberg on the occasion of the Kant Commemoration, February 1854.

On the Compensation of Impressions moving over the Retina.
By Sir DAVID BREWSTER, K.H., F.R.S.

The author stated that when, in railway travelling, they looked at the lines which the stones or gravel or other objects formed in consequence of the durations of their impressions on the retina, and quickly transferred the eye to the same lines further back, where the velocity was slower, the stones or gravel or other objects would, for an instant, be distinctly seen, just as rapidly revolving objects are seen in the dark when they are illuminated by an electric flash or the light of an exploded copper cap. A similar, but not the same, phenomenon will be seen when we look at the moving lines through a slit and quickly look away from the slit, so that the lines may be seen by indirect vision on a part of the retina not previously impressed. This class of phenomena may be best studied with a rapidly revolving disc, by quickly transferring the eye from the lines on the marginal part of the disc to those near the centre of rotation, where the velocity is less. When the marginal velocity is greatest, the point of compensation is nearest the centre, as might have been expected from the experiment in a railway carriage; but what could not, he thought, have been anticipated, was that the point of compensation was not in the same radius as the point to which the eye was first directed. The author explained this statement by means of a diagram which was exhibited. He had not been able to see the point of compensation close to the centre of rotation, where it doubtless must be, with a certain velocity, so that its locus must be in a curve.

On the Optical Study of the Retina. By Sir DAVID BREWSTER, K.H.,

F.R.S.

There were two structures in the retina (hexangular and quadrangular) that could be exhibited by optical means, the one by the successive impulses of light, and the other by the action of faint light entering the eye, or produced within it, either from the duration of a luminous impression, or from a local pressure upon the retina. The first of these structures was best seen by the light of a white cloud, through the slits or apertures of a revolving disc, placed midway between its circumference and its centre of rotation, in order to protect the eye from light which did not pass through the slits. When the disc revolved rapidly the field of view exhibited neither colour nor structure, but merely a diminution of light. When the velocity had reached a certain point, the field of vision became yellowish white, then yellow and bluish. Occasionally the yellow had the form of a rectangular cross, between the branches of which were four dark spaces. With a diminished velocity the whole field became uniformly blue, and was now covered with the hexagonal pattern formed by deep-black lines, the lines being darker at the place of the foramen centrale. As there are no fewer than eight different layers in the retina, it was of great importance to ascertain the functions which they individually performed in conveying visual impressions to the brain, and it was only by optical means that this inquiry could be conducted. The anatomist had ably performed his part with the aid of the microscope, and it was probably from the improvement of this instrument chiefly that we could expect any further discoveries, unless the morbid anatomy of the retina should connect certain imperfections of vision with the condition of certain layers of the membrane. When the eye was left in darkness, by the sudden extinction of a light, there were several points at the margin of the retina which retained the light longer than the rest. There could be no doubt that these effects were produced by structural differences. In the case of the foramen the difference had been recognized by the anatomist, and was proved by the remarkable phenomenon of Haidinger's brushes, and by other optical facts, such as the instability and superior brightness of oblique impressions on the retina. We had, consequently, an optical principle which enabled us to explain the quadrangular structure he had referred to. It was not improbable, when we looked at the complete structure of the retina, and even of its individual layers, that the structure of each of them might be exhibited optically.

On Binocular Lustre. By Sir DAVID BREWSTER, K.H., F.R.S. The author commenced by stating that some years ago it was observed by Professor Dove that when the right and left eye figures of a pyramid, or other mathe

matical solid, the one drawn on a white, and the other on a dark ground, were inserted in the stereoscope, the solid in relief appeared with a particular lustre. Prof. Dove described the lustre as metallic; and in another place, where he described the two diagrams as drawn, the one with white lines on a black ground, and the other with black lines on a white ground, he stated that the pyramid in relief “appears lustrous, as made of graphite." Other observers described the lustres differently, some as resembling ground glass, and others as like paper darkened with a blacklead pencil, while Professor Rood regarded it as "recalling the idea of highly polished glass." In order to explain this phenomenon, Professor Dove remarked "that in every case where a surface appeared lustrous, there was always a transparent, or transparent-reflecting stratum of much intensity, through which we see another body. It is therefore externally reflected light in combination with internally reflected or dispersed light, whose combined action produced the idea of lustre. This effect," he elsewhere added, "we see produced when many watch-glasses are laid in a heap, or when a plate of transparent mica or talc, when heated red-hot, is separated into multitudes of thin layers, each of which, of inconceivable thinness, is found to be highly transparent, while the entire plate assumes the lustre of a plate of silver." To these examples of lustre, produced by thin plates not in optical contact, or if in actual contact, having different reflective powers, were to be added the following pearls, mother-of-pearl, pearl-spar, and composite crystals of calcareous spar, and decomposed glass of all colours. The cause of these various kinds of lustre, and of that of metals, had always been well known, and when binocular lustre attracted the attention of philosophers, it was natural to ascribe it to the same cause. Professor Dove did this, and considered the dark surface in the one picture as the dispersed light, and the white surface as the regularly reflected light, the dark surface being seen through the white surface. This theory of binocular lustre, he had reason to believe, was not satisfactory. The phenomenon was first observed by himself in 1843, under conditions of different forms than those under which it was subsequently seen in the stereoscope. Having adverted to a paper "On the knowledge of Distance given by Binocular Vision," published by himself in 1844 in the Edinburgh Transactions,' he said that with his knowledge of the phenomena he could not adopt Professor Dove's explanation of the lustre seen in the stereoscope by the union of figures on dark and white, or differently coloured surfaces. In order to test this explanation by other means, he combined surfaces that had no geometrical figures upon them, and he found that binocular lustre was not produced. This experiment seemed decisive of the question. He was led to infer from it that the lustre observed in the combination of right and left eye figures of solids was not due to the rays from a dark surface passing through a lighter one to the eye, but to the effect of the eyes in combining the two stereoscopic figures, and to the dazzle occasioned by the alternating intensities of the two combined tints, the impression of one of the tints sometimes disappearing and reappearing. He referred to an article published by Professor Rood, of Troy, on his (Sir David Brewster's) "Theory of Lustre," and which he disavowed, not having adopted any "theory of lustre." He had merely started an objection to Professor Dove's theory of binocular lustre, and given an opinion regarding its cause; and as the simple experiment on which he founded that opinion had been made by others with a different result, he thought it right to re-examine the subject with the assistance of other eyes than his own, and had obtained results which might be of use to those who were disposed to study the subject more elaborately.

Binocular lustre was a species of lustre sui generis. It was a physiological, not a physical phenomenon, and had no relation whatever to those varieties of lustre which arose from the combination of lights reflected from the outer and inner surfaces of laminated, transparent, or translucent bodies. He assigned various causes for the physiological character of the phenomenon, and then added, "If binocular lustre arises from a physiological and not from a physical cause, we must look for this cause in the operations which take place in the eyes of the observer when binocular lustre is distinctly seen. These operations are of two kinds. First, in combining geometrical or other figures to represent solids whose parts are at different distances from the eye, the optic axes are in constant play, not only in varying the distance of their focus of convergence, to unite similar points at different distances in the two diagrams, but in maintaining the unity of the picture by

rapidly viewing every point of its surface. Secondly, when the two surfaces have different shades or colours, the retina of one eye is constantly losing and recovering the vision of one of them. Each optic nerve is conveying to the brain the sensations of a different tint or colour. The brain is therefore agitated sometimes with one of these sensations and sometimes with the other, and sometimes with both of them combined, and it is therefore not an unreasonable conclusion that, in the dazzle produced by this struggle of flickering sensations, something like lustre may be produced. In studying the subject of lustre there are some facts deserving of attention. In a daguerreotype, for example, of two figures in black bronze with a high metallic lustre, it is impossible by looking at either of the pictures to tell the materials of which they are made. No lustre is visible; but when the two equally shaded pictures are combined in the stereoscope, the lustre and true character of the material is instantly seen. Another instructive example is seen in the stereoscopic representations of a boy blowing a soap-bubble. The lustre of the watery sphere is not visible in either of the two pictures; but when they are combined, it is distinctly seen. In both these cases, and others of the same kind, tints of similar intensity are combined; and there is no ground fora ssuming that the two surfaces combined appear at different distances, and that the one is seen through the other, as in Professor Dove's theory.

Observations upon the Production of Colour by the Prism, the Passive Mental Effect or Instinct in comprehending the Enlargement of the Visual Angle, and other Optical Phenomena. By J. ALEXANDER DAVIES.

The communication was intended to show that the doctrine of the decomposition of light is not the only possible explanation of colour, but that two causes may, in the way of possibility, be assigned to its production, of which the other is, that the rays receive certain affections or dispositions by their transit through a prism or other media. It was not affirmed that the present doctrine, which of course implies previous combination or composition, is not the probable one, but only that the idea of its necessary exclusiveness, as the only one which can philosophically be maintained, is a philosophical error. The difficulty of imagining decomposition in some cases, as, for example, when the solar rays pass through a piece of smokeblackened glass, was referred to as affording some presumption for supposing that the production of colour by the prism is not occasioned by decomposition, and this especially when it is considered how difficult it is to conjecture how the prism effects the disintegration of the incident light. The equal difficulty appertaining to the hypothesis of disposition was also allowed; and it was shown that upon either explanation it must be granted that the incident rays pass to the second dyes of the prism, and back again to the first, before they are decomposed, or colours are otherwise produced, and that probably they arise from the backward transit of the rays, which is probably a species of retrogression. The phenomenon, that only the contours and internal lines and points of objects and pictures are coloured when seen through a prism, was accounted for by supposing that the rays proceeding from them are prevented from being recomposed by reason of the disturbance of the surrounding colour, which is not affected when seen through a prism, because the various rays are, by the law of chromatic aberration, united after being decomposed by it.. The comprehension of the visual angle, or the determination of the prolongation of the angular space, in every case of reflexion and refraction, was set down either to passive mental action or instinct, and this on the ground of there not being any physical barrier, and from the fact that single vision alone is sufficient to produce this effect.

The light proceeding from luminous objects was stated to be accompanied with colour, and not colour per se: and as regards the intensity of colour, it was concluded that, as an example, a thin mixture of Indian ink is caused either by the very thin distribution of black particles, or white or almost white ones, more or less closely compacted; supposing which to be the case, the mixture is darkened with every increase in their compactness; of which explanations the former was considered to be the correct one.

The fact that black polished surfaces, however great the approximative perfec