FORMATION OF IMAGE ON THE RETINA. 423 



in a focus on the retina. As every point is thus repre- 

 sented in its proper position relatively to others (except that 

 those which were above are now below, and vice versd), a 

 complete inverted image or picture of the object is formed 

 upon the retina. This is shown in fig. 210 ; where, for the 

 sake of convenience, three rays only are represented as issuing 

 from the centre and the two extremities of an object a b. These 

 rays cross each other at h, in the middle of the eye ; so that 

 those from a being brought to a focus at c, and those from 

 b at d, and all the other rays being refracted in the same 

 manner, a complete inverted picture of the object is formed at 

 the back of the eye. 



544. That this is really the case, may not only be inferred, 

 but proved. If the eye of a Rabbit be removed from its 

 socket, and cleansed of the muscles, fat, &c., adherent to its 

 back part, and a candle be then brought in front of it, the 

 transparency of the sclerotic coat will allow the image of the 

 candle that is formed upon the retina to be distinctly seen. 

 Or, if we take the eye of a Sheep or an Ox, and after cleans- 

 ing it in the same manner, we cut out from the back of it- a 

 portion of the sclerotic and choroid coats, covering the part 

 of the retina thus left bare with a piece of tissue-paper (for 

 the purpose of keeping -in the vitreous humor, without 

 interrupting our view of the image), a distinct but inverted 

 miniature picture of all the objects in front of the eye will 

 be seen at its back. It is necessary in these experiments 

 that the eyes should be taken from animals recently killed ; 

 as the cornea and humors soon lose their transparency, and 

 the distinctness of the picture is consequently impaired. 



545. The black pigment, which is situated immediately 

 behind the retina, that is, in contact Avith its external 

 surface, is destined to absorb the rays of light immediately 

 that they have passefl through the retina j so as to prevent 

 them from being reflected from one part of the interior of 

 the globe of the eye to another, which would cause a great 

 confusion and indistinctness in the picture. Hence it is that 

 in those individuals (both among Man and the lower animals) 

 in whose eyes this pigment is deficient, vision is extremely 

 imperfect. The eyes of such individuals (termed Albinos) 

 derive from the absence of pigment a very peculiar appear- 

 ance. The iris does not possess its ordinary colour ; but, 



