IIO THE RETINA. 



retinal vessels on parts of the bacillary membrane, which are not accustomed 

 to receive them. If the source of light moves, the shadows move with it, 

 and in the same apparent direction. The motion of the retinal image (i) can 

 be measured ; that of the light (2) can also be measured, and the distance (3) 

 of the vessels from the bacillary membrane is known. If from 2 and 3, I be 

 calculated, it will be found to agree with the measurement. 



The fovea centralis is more perfect structurally and func- 

 tionally than any other part of the retina. Accordingly, as it 

 lies approximately in the axis of the eyeball, objects which 

 lie in the prolongation of this axis (the visual line) i.e. those 

 on which the eye is fixed, are seen more distinctly than any 

 others. Thus, two objects so near together that the straight 

 lines leading from them to the crossing point of the eye, 

 meet each other at an angle of 60" or 70" (and of which con- 

 sequently the retinal images are at most 0*005 millimeter 

 from each other), can be distinguished as two. If the 

 images fall on the retina outside of the fovea, they must 

 be at least a millimeter apart, in order to be distinguished. 

 If the rays from two objects at the same apparent height 

 meet at 15, and the eye is fixed on the one nearest the 

 middle plane of the body, the other (provided that its 

 retinal image does not measure more than 1*5 millimeter) 

 is not seen, for its image falls on the entrance of the optic 

 nerve, the so called " blind spot." 



Retinal Purple. The external layer of the bacillary membrane (outer joints 

 of the rods and cones) is infiltrated with a red colouring matter, which in the 

 eye removed from the body, remains unaltered so long as the retina is in the 

 dark, or is exposed only to yellow light. On exposure to ordinary or blue 

 light it disappears, but can be restored even after the cessation of the circula- 

 tion, by contact in the dark with the pigment epithelium. During life it is 

 alternately destroyed and reproduced according as the eye is exposed, or not 

 to light. 



Excitability of the retina. Process of excitation in the 

 retina. As with respect to other excitable structures, so in 

 the case of the retina, we may best distinguish between the 

 excitation and the physiological effect which it occasions 

 (sensation of light), by studying their time-relations. We 

 learn by observation, (i) that the sensation of light produced 



