Mr. kVheatstone has also shown that, if similar images, differing only in magnitude, are presented to analogous parts of the retina?, the resultant idea is that of an image appa rently intermediate in size between them.% Foucault and I. Regnault have employed the stereoscope to determine whether com plementary retinal images produce the im pression of white on corresponding points of both eyes. Two complementary rays, ob tained by chromatic polarisation, were thrown in a horizontal direction on the mirrors of a stereoscope, and reflected from them on the screens attached at the sides, so that two small disks of paper were coloured by them. At first one or other complementary colour was alternately seen after a time, however, the two impressions united to form white ; and when the eyes had once become accus tomed to this, a whole series of complemen tary colours could be introduced successively, and nothing but white was seen during the entire experiment.
From the following interesting experiments by Sir David Brewster, it appears that we give solidity and relief to plane figures by a suitable application of colour to parts that are placed at different distances from the eye. If we look with both eyes through a lens about two and a half inches in diameter, at an ob ject having colours of different refrangihilities, as a red rose among green leaves, the two colours will appear at different distances from the eye of the observer. In this experiment we are looking through the margin of two semi-lenses, or virtual prisms, by which the more refrangible rays are more refracted than the less refrangible rays. The doubly coloured object is thus divided into two, as it were, and the distance between the two blue por tions is as much greater than the distance between the two red portions (red and blue being supposed to be the colours) It s twice the deviation produced by the virtual prism, if we use a large lens or two semi-lenses, or by the real prisms if we use prisms. The images of different colours being thus sepa rated, the eyes unite them, as in the stereo scope, and the red image takes its place nearer the observer than the blue, in the same manner as the two nearest portions of the dis similar stereoscopic figures stand up in relief at a distance from their more remote por tions. The reverse of this will take place, if a concave lens be used, or if the refracting angles of the two prisms be turned inwards. The modified stereoscope has very recently been applied to photographic purposes with the happiest results.
Adaptation to distance. — On no subject connected with the physiology of the eye has there been a greater diversity of opinion than on the mode in which adjustment to distance is performed. That such adjustment is necessary, is proved by the simple experiment of looking between the fingers held about eight inches from the eye, at a distant object. When the distant object is seen distinctly, the fingers will be seen indistinctly; and if we look at the fingers so as to see them quite distinctly, the distant object will be indis tinct. Our space will not admit of our doing more than glancing rapidly at the theories which have been advanced, and eagerly sup ported, to account for this alteration in the focus of the eye. By Bay-le, Rohault, Home, Olbers, and Schroeder Vanderkolk, alteration in the form of the eye by means of the ex ternal muscles was supposed to be the medium of adjustment. The movements of the iris
have had their supporters, of whom the prin cipal were Mile and Pouillet. Ramsden and Sir y. vcrard Home regarded a change in the convexity of the cornea as the medium of adjustment. By John Hunter and Dr. Young (who devoted much time to the inquiry), elongation and shortening of the axis of the lens, through a contractile power inherent in the lens itself', were supposed to be the cause; and lastly, it was referred by many authori ties, including Kepler, Scheiner, Camper, and Porterfield, to the movement of the lens by means of the ciliary processes. Porterfield was probably the first who hit upon the true explanation, by referring the adjustment to the action of the ciliary body upon the crys talline, and by distinctly asserting the mus cularity of the ciliary body. In reply to the arguments of De la Hire (who maintained that at whatever distance objects were placed, the eyes never altered their focus), Porter field acutely observes : " This author main tains that it is impossible the crystalline can change its situation, because the ciliary liga ment is not muscular, and consequently has no power of contraction : and of this opinion are likewise a great many anatomists, and in particular Hovitet : but it appears that all of them have been led into this mistake by an unjust notion they have entertained about the colour of muscles. Every body knows that our muscles are generally of a red colour; but it does not from thence follow that what is not red is not inusculous. The muscular fibres of the guts and stomach have scarce anything of redness in their colour ; and it is also certain that the pupil does contract and dilate itself according as objects are more or less lumi nous, and yet none of the fibres which perform that action are in the least red. Whence it t011ows that the fibres of the ligamention ciliare are not to be deprived of a power of con traction because of a colour different from what generally obtains in other muscles ; nor are we to be surprised that so many accurate anatomists, after a careful examination of this process, have not scrupled to affirm it to be truly muscular." Mons. Pouillet has advanced the hypothesis that, by the peculiar conformation of the lens, near objects are seen through the medium of the rays passing through its centre, and distant objects by means of the circumferential rays. He describes the crystalline lens as made up of strata, differing in curvature and density, so that its section exhibits a series of concen trical ellipses having varying excentricities, the internal strata being more curved and more dense than the external ; whence the rays which pass from the latter converge to a more distant point than those from the former. According to this theory, the crystalline lens has many different foci, and the effect should be, that when a pencil of rays falls upon it, those rays which are near the axis of the pencil, and therefore near the centre of the lens, are brought to a shorter focus than those which are near the border ; whence near objects would, says M. Pouillet, be seen by means of the central rays, and distant objects by means of those rays which fall near the borders of the crystalline lens.