OPTICS 271 



the point c of the retina will remain dark and we see a dark spot 

 in the visual field in the direction cd (prolongation of a straight 

 line drawn from c through the nodal point). The shadows cast by 

 the retinal vessels upon the retina are perceived as black ramifying 

 lines in the visual field. If now the light is moved from a to a, 

 the apparent position moves from d to d, as illustrated in the figure. 

 From the extent of both movements the distance between g and c 

 may be calculated, and it has been found that this corresponds to 

 the distance between the vascular nerve-fibre layer and the layer of 

 rods and cones. 



The place where the optic nerve enters the eye [blind spot] is 

 not sensitive to light, because at that place no rods and cones are 

 present. This is demonstrated as follows : 



FIG. 34. 



Close the left eye, and with the right eye look at the cross a 

 (Fig. 34) while the page is held about 25 cm from the eye. The 

 black circle is then invisible because its image falls upon the blind 

 spot. 



It has been supposed that in the sensitive apparatus a substance, 

 the so-called visual substance, is decomposed, and that this decom- 

 position calls forth the stimulation. 



I. Objectively demonstrable changes in the retina pro- 

 duced by light. Changes in the retina produced by light 

 have been demonstrated, but their relation to the sensation 

 of light is not known. 



i. Visual purple (rhodopsin) is a red pigment found in 

 the external elongation of the rods. This is bleached by 

 light, the color being restored in the dark and also in the 

 excised eye (in rabbits in one half-hour, in frogs in one to 

 two hours). The restoration proceeds from the pigment 

 epithelium. Visual purple cannot be the only visual sub- 

 stance, for it is not present in the macula lutea of man, the 

 place of direct vision (see page 277). 



The visual purple can be extracted from a fresh retina by an 

 aqueous solution of bile salts. This must be done in the dark or 



