238 SENSE OF SIGHT. 



ABC and its image on the retina, is the common camera obscura. The 

 eye is, therefore, more complicated and more perfect than this simple 



Fig. 114. 



Camera Obscura. 



instrument : the cornea, with the aqueous and vitreous humours, is 

 added for the purpose of concentrating the light on the retina; the lat- 

 ter, in addition, affording a large space for the expansion of the retina, 

 and preventing the organ from collapsing. In the operation for cata- 

 ract by extraction, which consists in removing the lens through an 

 opening made in the lower part of the cornea, the aqueous humour 

 escapes, but is subsequently regenerated. If, however, too much pres- 

 sure be exerted on the ball, to force the crystalline through the pupil 

 and the opening in the cornea, the vitreous humour is sometimes 

 pressed out, when the eye collapses, and is irretrievably lost. 



Experiments have been instituted on this subject, the results of 

 which are even more satisfactory than the facts just mentioned. 

 These have been of different kinds. Some experimenters have formed 

 artificial eyes of glass, to represent the cornea and crystalline, with 

 water in place of the aqueous and vitreous humours. Another mode 

 has been to place the eye of an ox or a sheep in a hole in the shutter 

 of a dark chamber, having previously removed the posterior part of 

 the sclerotica so as to permit the images of objects on the retina to be 

 distinctly seen. Malpighi and Haller employed a more easy method. 

 They selected the eyes of the rabbit, pigeon, puppy, &c., the choroid 

 of which is nearly transparent ; and, directing the cornea towards 

 luminous objects, they saw them distinctly depicted on the retina. M. 

 Magendie 1 repeated these experiments by employing the eyes of albino 

 animals, as those of the white rabbit, white pigeon, white mouse, &c., 

 which afford great facilities, the sclerotica being thin, and almost 

 transparent ; the choroid, also, thin, and when the blood, which gives 

 it colour, has disappeared after the death of the animal, offering no 

 sensible obstacle to the passage of light. In every one of these ex- 

 periments, external objects were found to be represented on the natu- 

 ral or artificial retina in an inverted position ; the image being clearly 

 defined, and with all the colours of the original. Yet how minute 

 must these representations be in the living eye ; and how accurate the 

 mental appreciation, seeing, that each impression from myriads of 

 luminous points is transmitted by the retina to the encephalon, and 

 perceived with unerring certainty ! 



1 Precis Elementaire, i. 70. 



