OPTICS 269 



a light F, which is placed at the side of the head, into the 

 eye B, a part of the rays reflected by the background of 

 the eye leave B and pass through the aperture in the mirror 

 into the eye A. The eye A then sees the background of 

 the illuminated eye B. If both eyes are emmetropic and at 

 rest, the light reflected by B passes out in parallel rays and 

 A sees the back of B erect and enlarged. If the light leaves 

 B in convergent rays (in case B is accommodated or myopic 

 ras represented in Fig. 31), the image of the retina can be 

 seen erect by placing a concave lens, making the rays 

 parallel or divergent, between A and B. If a convex lens 

 of sufficient strength is placed between B and 5, the reflected 

 rays form, at some point between 5 and the lens, a real and 

 inverted image of the background of B which can be 

 observed by the eye of the observer A (observation of in- 

 verted image). 



In the albino, diffused light enters the eye through the trans- 

 parent iris which contains no pigment ; this light can illuminate the 

 background of the eye. If this diffused light is prevented from 

 entering the eye, as by placing a diaphragm in front of the eye, the 

 background of such an eye also appears dark. 



2. THE STIMULATION OF THE RETINA BY LIGHT. 

 SENSATION OF LIGHT 



The elements of the retina sensitive to light are the cones 

 and rods. From them the stimulation passes through the 

 nervous portion of the retina and through the optic nerve to 

 the brain. 



The nerve structure of the retina is found in the connective-tissue- 

 supporting elements and is composed of three neurons placed one 

 behind the other (see Fig. 32). They are : 



(1) Neuro-epithelial cells, i.e. the cones (0) and rods (.$) with 

 their nucleated portion (cone and rod granules, z^ and .$*,), which 

 are joined by processes to the outer processes of the 



(2) Si-polar cells (<5). The inner processes of the bi-polar 

 cells join the protoplasmic processes of the 



(3) Ganglionic cells (gj, whose axis-cylinders form the optic 

 fibres. 



In addition to these, still other cells and centrifugal nerve fibres 

 have been found in the retina, but their function is not known. 



