678 SPECIAL SENSES 



the retina to light, on account of the excessive quantity of visual purple 

 which has accumulated in the dark ; and that distinct vision is restored 

 when the retina is bleached to a yellow, which seems to be the most 

 favorable condition for the exact appreciation of visual impressions 

 under full illumination. On the other hand, it requires time for the eye 

 to become accustomed to a dim light; and during this time the yellow 

 is changing to purple. Investigations of the absorption-spectra of the 

 purple and yellow have shown that the purple allows the actinic rays to 

 pass perfectly, while the yellow completely absorbs these rays. The 

 existence of visual purple seems to be most favorable to the imperfect 

 and shadowy vision which occurs under dim illumination, when the 

 exact appreciation of minute details is impossible. In the condition 

 known as night-blindness, it is probable that the visual purple has be- 

 come exhausted beyond the possibility of prompt restoration such as is 

 normal ; and persons so affected can not see at night, although minute 

 vision under a bright light may not be affected. In certain cases of 

 this kind, the normal conditions may be restored by a few days' seclu- 

 sion in the dark. What is called functional night-blindness frequently 

 occurs in sailors during long tropical voyages, and is due to the exces- 

 sive action of diffused light on the retina. 



Change of visual purple to yellow is readily effected, but the further 

 change to white is slower and more difficult. Conversely, change from 

 white to yellow is slow and change from yellow to purple is compara- 

 tively prompt. One use of the colors, purple and yellow, seems to be to 

 accommodate the retina for vision under different degrees of illumina- 

 tion. The purple adapts the eye to a feeble illumination, and the 

 yellow, to a full illumination. This being the case, it is manifestly 

 proper to speak of a visual yellow as well as of visual purple. 



That the accommodation of the eye to different degrees of illumina- 

 tion is due to changes in the colors produced by the pigmentary layer 

 of the retina and not to different degrees of dilatation of the pupil, is 

 shown by the fact that a person does not see better in the dark when 

 the pupil has been dilated by atropin. In a very dim light there is no 

 possibility of exact accommodation for near objects, which, when small, 

 can not be seen distinctly; and the contraction of the pupil that attends 

 accommodation for near vision does not occur. It is possible that under 

 dim illumination, parts outside of the fovea, that are insensible to vision 

 under a bright light, receive visual impressions. Under these conditions 

 the pupil is dilated and rays impinge on portions of the retina not used 

 in direct vision. A natural extension of this idea would confine distinct 

 vision and the appreciation of minute details to the action of the fovea 

 centralis, in which there is no visual purple, other parts of the retina, 



