H50 



VISION. 



on each retina, becomes less and less as the 

 object is seen at a greater distance from the 

 eyes, and entirely ceases when it is so distant 

 that the optic axes are parallel while regarding 

 it. We see, with both eyes, all objects beyond 

 this distance precisely as we see near objects 

 with a single eye ; for the pictures on the two 

 retinae are then exactly similar, and the mind 

 appreciates no difference, whether two iden- 

 tical pictures fall on corresponding parts of 

 the two retinae, or whether one eye is im- 

 pressed with only one of these pictures. A 

 person deprived of the sight of one eye sees 

 therefore all external objects, near and remote, 

 as a person with both eyes sees remote objects 

 only : but that vivid effect, arising from the 

 binocular vision of near objects, is not per- 

 ceived by the former, who, to supply the 

 deficiency, resorts to other means of acquiring 

 more accurate information ; and of these the 

 motion of the head is the principal. 



Mr. VVheatstone has also shown that, if 

 similar images, differing only in magnitude, 

 are presented to analogous parts of the retinas, 

 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 ol 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 refrangibilities, 

 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) us twice 



* Sir David Brewster disputes this and several 

 other deductions of Mr. Wheatstone, in an able paper, 

 " On the Law of Visible Position in Single and 

 Binocular Vision," Transactions of the Royal So- 

 ciety of Edinburgh, vol. xv. 



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 Bayle, 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 Everard 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 Hovius : 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 



