THE SENSES. 537 



four hours in a four per cent, watery solution of potassium-alum. The 

 last operation is for the purpose of giving to the retina a greater con- 

 sistency, so that it may be removed from the eyeball without lacera- 

 tion. When the hardening is complete the retina is removed, and placed, 

 with its posterior surface uppermost, upon a porcelain capsule of suit- 

 able convexity; when the images of the window-panes are seen in 

 white, with the intervening bars and the surrounding spaces purple- 

 red. On exposure to daylight the images disappear, owing to the 

 bleaching of the whole retina ; but if, while still in the dark chamber, 

 it be thoroughly desiccated, its color becomes comparatively indestruc- 

 tible, and the optograms remain visible in daylight for many hours. 



Notwithstanding the evident importance of the retinal red, and its 

 sensibility to the influence of light, it is not immediately essential to 

 the act ef vision. This is manifest, in the first place, from the fact 

 that, in the human eye, it is absent from the macula lutea and fovea 

 centralis ; that is, from the spot of greatest retinal sensibility and most 

 distinct vision. Kiihne has furthermore demonstrated that frogs whose 

 retinas have been completely bleached by continued exposure to direct 

 sunshine are still capable of vision. Under these circumstances the 

 retinal red is regenerated, even in the dark, somewhat slowly, and 

 does not begin to show itself under half an hour (page 535). During 

 this period, therefore, the animals have no appreciable red in the retinal 

 tissue ; and yet they quickly distinguish moving objects, and can even 

 capture flies in their usual manner with readiness and precision. They 

 also show a capacity for distinguishing colors, and in both these par- 

 ticulars exhibit a marked contrast with frogs which have been blinded 

 by extirpation of the eyeballs. 



It is accordingly quite uncertain in what way the coloring matter 

 of the retina is subservient to sight. It may be supposed that by its 

 transformation under the influence of light it supplies some material 

 for the continued nutrition of the nervous elements; and that this 

 secondary material is in turn consumed during the act of vision. But 

 so far as our present knowledge extends, there is no satisfactory evi- 

 dence in regard to its mode of action. 



Physiological Conditions of the Sense of Sight. The eye, so far as 

 regards its physical structure, is an optical instrument, composed of 

 transparent and refracting media, a perforated diaphragm, and a dark 

 chamber, all of which act upon luminous rays according to the same 

 laws as the corresponding parts in a telescope or a camera ; and the 

 accuracy of their adjustment is one of the first requisites for the exer- 

 cise of sight. The eye is also movable in various directions ; and cer- 

 tain of its internal parts are under the control of muscular tissues, 

 which contribute to its action. It is furthermore a double organ ; and 

 impressions may be , acquired by the use of both eyes which cannot 

 be received from one alone. Finally, the sensibility of its nervous ele- 

 ments is liable to modifications, which influence the nature and intensity 



