LIGHT-SENSE 45 



long time, the congenitally total color blind, the mammals, 

 already referred to, who have only rods in their retinae, and 

 nocturnal birds, all have a dread of light that is, photo- 

 phobia. They are all temporarily or permanently dependent 

 for vision on perceptions received through their rods and 

 the visual purple. 



Those who have been exposed for a long time to glare, 

 like sailors becalmed on a tropical sea, those who have the 

 congenital affection which we call 'night-blindness," or the 

 acquired affection which is spoken of as 'retinitis pig- 

 mentosa," and diurnal birds, all have a dread of darkness; 

 that is a condition which I suggest may be conveniently 

 termed "scotophobia." They are all temporarily or per- 

 manently dependent for vision on perceptions received 

 through their cones. 



Xo other photo-chemical substance has been discovered 

 in the retina besides the visual purple. It becomes, there- 

 fore, natural to inquire whether some reaction other than 

 chemical is not produced in the cones capable of originating 

 a nerve impulse on stimulation by light. 



Stort and Englemann 18 have described how a shortening 

 of the inner segment of the cones takes place by the action 

 of light and an elongation in darkness. On exposure to 

 light, therefore, the outer segments of the cones are drawn 

 toward the outer limiting membrane and away from the 

 pigment-cells, while in darkness the opposite movement 

 takes place. This effect has been observed in the retina 

 of all animals hitherto examined, viz., fish, amphibians, 

 reptiles, birds and mammals, including man. The part 

 which moves actively is that which in its optical and chemical 

 properties is like protoplasm, viz., the part of the cone between 

 the ellipsoid and the cone nucleus, and to this they give the 

 name of "cone-myoid." This movement in the cones may 



