each hand one prism, the two prisms having their thin edges towards each other, we look at the window from the opposite end of the room, we see first two windows with their vertical lines bent in contrary directions; but by inclining gradually the optical axes, we can converge them until the two images coalesce, and we see only one window: as soon as they coincide the lateral curvature of the vertical lines ceases, and they are bent projectively from back to front: we have then the illusion of a window concave towards the room, such as it would appear reflected by a concave mirror.

ON THE TRANSMISSION OF THE ACTIVE RAYS OF LIGHT THROUGH THE EYE, AND THEIR RELATION TO THE YELLOW SPOT OF THE RETINA.

The following is an abstract of a paper, communicated to the Edinburgh Philosophical Journal, on the above subject, by Dr. Wilson. Hartmann, as long ago as 1849, stated, that the eye arrests the chemical radiations which accompany the more refrangible rays of light. Dr. Wilson not being wholly satisfied with this statement, determined to re-examine the whole subject, and to test conclusively the question, whether the eye can transmit the chemical rays of light. An ox eye was prepared by cutting away the sclerotic coat, until the choroid came into view; a circular aperture of oneeighth of an inch in diameter was then made through the membranes and the retina, which laid bare the vitreous humor at a point opposite to that where the light enters. The eye was then supported in the brass mountings of a photographic lens, resting at the posterior end on a ring of cork, which fitted tightly into the tube, and retained in front by a diaphragm, so as to permit the cornea to protrude. From the arrangement of the fittings no light, except that which passed through the eye, could enter through the camera. A collodion glass plate being placed in the box and the camera being properly directed, an image was developed after an exposure of fifteen seconds. Other experiments subsequently made under a variety of conditions, such as would seem to obviate every objection, give similar results; and from these it appears certain, that the chemical or active rays of light are not arrested in their passage across the chamber of the eye.

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An important question to be next considered is, what change do the actinic rays undergo when they fall upon that particular portion of the human retina which anatomists distinguish as the "yellow spot." This spot," almost peculiar to man, presents a diameter of of an inch, and occupies the bottom of the eye, in the exact axis of its transparent humors. It is more transparent than the rest of the retina, and has long been recognised as the seat of the most perfect vision in the eye of man. Now this colored medium of the eye must produce a certain effect on the light which reaches it, and on the actinic rays which traverse it. Prof. Goodsir has shown, that it is not merely the case that light traverses the retina to the choroid, and is then reflected so as to return through the retina, but it is only the rays thus returned which produce a luminous sensation. The light, therefore, which traverses the yellow spot, and is then reflected forwards on the choroidal extremities of the optically sensific constituents of the

retina, must be deprived, to a greater or less extent, of its actinic rays (by passing through the yellow spot) before it determines a luminous sensation, unless the portion of the retina under consideration differs from all other transparent media known to us, in not arresting the chemical rays. If it be not in this respect exceptionable, then the theory of perfect human vision may be simplified by the exclusion from consideration of the actinic rays; and one use of the yellow spot, for which it has hitherto baffled physiologists to find a use, may be to extinguish these radiations. It may be further remarked, in reference to the absorption of the actinic rays by the yellow spot, that the views of those who have described visual impressions on the retina as phenomena of the same kind as photographic impressions on chemically prepared surfaces must fall to the ground, if the actinic rays of light are stopped before reaching the optically sensific constituents of the retina. The similar opinion, also, that “spectral vision" and other abnormal peculiarities of sight, are phenomena of the same kind as the development (as it is technically called) of latent photographic images, must, for the reason mentioned, be abandoned. It will still of course be competent to compare normal and abnormal vision with photographic effects, as phenomena displaying analogy, though not affinity.

One other relation of the retina to light may be referred to. If only these rays which are reflected from the choroid produce by their impact on the retina the objective perception of light, and if the depth of tint of the yellow spot be considerable and its color at once homogeneous, then perfect vision must be exercised by yellow, and not by white light. But if this be the case we should be unconscious of red and blue when seeing best, or at least should receive from them an impression very different from that which they occasion when they affect the general surface of the retina.

NEW FORM OF TELESCOPE.

At the American Association for the Promotion of Science, Mr. Alvan Clarke, of Cambridge, gave a description of a new instrument of his own invention, for measuring the distance apart of stars too distant to be brought into the field of view of a telescope. Within a year from the first thought of the instrument entering his mind, he had built a telescope of six inches aperture, and 103 inches focal length, mounted it equatorially, governing its motion by Bond's spring-governor clock, provided the two eye-pieces, and as a substitute for a filar-micrometer, arranged a mode of using pieces of glass ruled with a ruling machine. Experiments have demonstrated the feasibility of using the two eye-pieces in this way, and of obtaining by them very accurate measures of the distances of stars, which are from three to one hundred minutes of space apart. The success of the instrument was, however, greatly due to the spring-governor which keeps each star upon the wire accurately bisected.

Professor Pierce said there were two other things besides the double eyepiece in this instrument, to which attention should be directed. The eyepiece must be tested by experiments, but the new mounting of the telescope, a modification of the Munich, was exceedingly beautiful, more so than even

the Munich, and vastly superior in convenience and value. The springgovernor also was put into the best condition for good action there, the heavy mass of the telescope acting directly as a balance wheel, and controlling all irregularity of movement. In short, the instrument satisfied all reasonable requirements for equatorial mounting.

KOBELL'S STAUROSCOPE.

At the German Association for the Promotion of Science, Dr. Grailich exhibited Professor Kobell's Stauroscope, for the investigation of the optical properties of mineral substances. The principle of this very simple apparatus consists in the disappearance of the dark cross shown by plates of calcareous spar, when placed between two tourmalines set across, by the interposition of any crystallized substance whose principal section of elasticity is not coincident with the polarizing planes of tourmaline. By turning the interposed plate of crystallized substance, a position of it is found in which the principal sections of elasticity coincide with the polarizing planes of the apparatus, and this coincidence is manifested by the reappearance of the dark cross.

GEOLOGICAL SPECIMENS FOR MICROSCOPIC PURPOSES. At the recent meeting of the German Association for the Promotion of Science, Professor G. Rose exhibited Dr. Oschalz's collection of lamellæ of minerals and rocks, reduced to a degree of thinness sufficient to render them transparent, and fit for microscopic investigation. This mode of investigation has led to the observation of many interesting facts respecting the crystalline structure of simple minerals, and the real constitution of composite substances, such as basalt, solenite, porphyry, &c. These plates, overlaid with Canada balsam, are placed between two pieces of glass, so as to be easily brought under the microscope. The price of the set of seventy-three plates is thirty-five thalers (about 51. 8s.).

ON THE PERMANENCE OF PHOTOGRAPHIC PICTURES.

The Committee of the English Photographic Artists, appointed to consider the question of the fading of photographic pictures on paper, have submitted a first report as follows:

The committee have unquestionable evidence of the existence of photographs which have remained unaltered for more than 10 years, prepared by salting plain paper with a chloride, afterwards making it sensitive with either nitrate or ammonio-nitrate of silver, fixing with a freshly made solution of hyposulphite of soda, and washing with water; also of positives produced by Mr. Talbot's negative process.

They have not been able to obtain evidence of photographs having been prepared at all upon albumenized paper, or colored with a salt of gold, or fixed with "old hypo," so long as ten years. They have, however, ample evidence of the existence of unaltered photographs so prepared five, six, and seven years ago.

They have not found that any method of printing which has been commonly followed, will necessarily produce fading pictures, if certain precautions be

adopted, nor have they evidence that any method which has been adopted, will not produce fading pictures unless such precautions are taken.

Causes of Fading.-The most common cause of fading has been the presence of hyposulphite of soda, left in the paper from imperfect washing after fixing.

The committee think it right to state, that they have been unable to find any test to be relied upon, which can be used to detect a minute portion of hyposulphite of soda, in the presence of the other substances which are obtained by boiling photographs in distilled water and evaporating to dryness; yet they have no doubt of the truth of the above statement, from the history given of the mode of washing adopted.

The continued action of sulphureted hydrogen and water will rapidly destroy every kind of photograph; and as there are traces of this gas at all times present in the atmosphere, and occasionally in a London atmosphere very evident traces, it appears reasonable to suppose that what is effected rapidly in the laboratory with a strong solution of the gas, will take place also slowly but surely in the presence of moisture, by the action of the very minute portion in the atmosphere.

The committee find that there is no known method of producing pictures which will remain unaltered under the continued action of moisture and the atmosphere of London.

They find that pictures may be exposed to dry sulphureted hydrogen gas for some time with comparatively little alteration, and that pictures in the coloration of which gold has been used, are acted upon by the gas, whether dry or in solution, less rapidly than any others.

They also find that some pictures which have remained unaltered for years, kept in dry places, have rapidly faded when exposed to a moist atmosphere. Hence it appears that the most ordinary cause of fading may be traced to the presence of sulphur, the source of which may be intrinsic from hyposulphite left in the print, or extrinsic from the atmosphere, and in either case the action is much more rapid in the presence of moisture.

Mode of Mounting Photographs.-The committee find that, taking equal weights, dried at a temperature of 212°, of the three substances most frequently used, viz. gelatine, gum, and paste, the latter attracts nearly twice as much moisture as either of the former, and as in practice a much smaller weight of gelatine is used than of gum, gelatine appears to be the best medium of these three; and the committee have evidence of fading having in some cases been produced by the use of paste.

In illustration of some of the circumstances alluded to above, the committee think it well to mention some instances of prints at present in their possession:-Out of several prepared together in 1844, three only were unaltered, and these were varnished soon after their preparation, with copal varnish. Half of another print of the same date was varnished, and the other half left; the unvarnished half has faded, the varnished part remains unaltered. Three pictures were prepared in 1846, all at the same time, with the same treatment; when finished, one was kept unmounted, the other two were mounted with flour paste at the same time, one of these latter having been

first coated with Canada balsam; at present the unmounted one and the one protected by the balsam are unchanged, whereas the other has faded.

A picture prepared in 1846 was so exposed that the lower part of it became wet by rain; at present the part so wetted has faded, while the rest of it remains unaltered. Several pictures were prepared and mounted about ten years ago, and kept in a dry room for about three years without any change, after which they were placed in a very damp situation, and then faded decidedly in a few months.

IMPROVEMENTS IN PHOTOGRAPHY.

Taking Photographic Images under Water.-Mr. Penny, of England, publishes in the Journal of the Society of Arts, the following account of his experimental researches in taking photographic images under the surface of water.

A box, made as nearly water-tight as possible, is prepared large enough to enclose the camera. This box is fitted, in part, with a piece of plate glass, and on the outside is a wooden shutter, heavily leaded, and which is raised by a string attached to it and communicating with a boat.

On each side of the box is an iron band, terminating in a screw, and projecting beyond the back, which is loose, and fitted with an iron bar, having a hole at each end, through which the screws of the band pass, and thus the back is screwed down tight against the body of the camera by means of a nut; the inner surface of the back is padded, so as to make the camera box, as far as possible, water-tight, when the back is screwed into its place.

The box is fixed to an iron tripod, and a band, with an eye on its upper margin, is passed round both camera, box, and stand; to this eye is attached the rope that lowers the camera to the sea bottom, and by which it is raised. This is the whole of the apparatus employed.

The first thing to be accomplished is to focus the camera, which process is thus described by Mr. Penny:

"The camera is placed in the box on the shore, and a view is focussed, taking as the foreground an object at ten yards' distance. This I did with the view now sent, but I fear it is too much. I then fix the stand by means of a triangular wooden frame forced up between the legs of the tripod stand, and which is prevented from slipping down by being attached to the top of the tripod by a line; this keeps the camera frame the exact distance from the ground that it was when focussed on land, and the camera being focussed for the same distance, it stands to reason that, provided the optical and chemical properties are the same, we shall obtain a similarly good picture.

"The next thing to be done is to prepare the plate and inclose it. The plate is prepared with collodion, in the usual way, under the tent. It is then placed in the camera (my camera used for this purpose takes a plate five inches by four inches). I then take the camera to the box and stand, and throw a black cloth over all. I examine the shutter in front of the camera box to see that it is tight; then, uncapping the camera under the cloth, I place it in the box, and finally draw up the slide. I then push the camera completely into the box, until the front of the lens presses against the plate glass front of the box,