LECT. XX. SEAT OF THE IMAGE FORMED. 367 



black pigment, which absorbs all the rays, which would 

 otherwise be again reflected within, and thus disturb the 

 distinctness of the image. All optical instruments have this 

 arrangement : thus, the tubes of telescopes and of micros- 

 copes are blackened in the interior. 



Seat of the Image formed. A very simple experiment 

 serves to show that images are formed at the bottom of the 

 eye, upon the retina. It consists in placing, in a dark 

 room, the eye of an Albino rabbit before the flame of a can- 

 dle placed at a proper distance : as the sclerotica in these 

 eyes is semi-transparent, we can distinctly see the image of 

 the flame inverted upon this membrane. Here is the eye 

 of an ox, whose sclerotica has been pared and rendered 

 almost transparent : each of you can see the image of the 

 flame, which I hold before it, inverted upon this membrane. 

 If we calculate by means of the formula for convex lenses, 

 taking into consideration the dimensions and refracting 

 powers of the different parts of the eye, we find that if an 

 object be placed at about 30 centimetres [11-8 English 

 inches] from the eye, its rays at this distance fall upon 

 the eye with the necessary degree of divergence for them 

 to converge to a focus upon the retina. From all this, 

 therefore, it is natural to conclude that vision, that is, the 

 sensation of a body which transmits luminous rays to our 

 eye, is owing to the modification effected in the retina by 

 the luminous rays brought together upon all the points of 

 this membrane, where the image of the body is formed ; 

 and to the transmission of this modification to the seat of 

 perception, by means of the optic nerve. In whatever way 

 the retina is excited, the sensation experienced is always 

 that of light : thus the passage of electricity, a blow, and 

 pressure of the eye, and, consequently, that of the retina, 

 produce luminous impressions there; so that the nerves 

 of the senses, when excited, are each susceptible of one 



