HISTOLOGY OF THK RETINA 



547 



It will be shown below that the retina when adapted for vision in 

 dim light is not only colour-blind, but also that the gr^en rays of the spectrum 

 appear to have to it the maximum luminosity. It will be also shown that 

 the rod containing parts of the retina, where visual purple is to be found, 

 are the only regions that react to light of low intensity. Moreover the 

 curve which represents the rate of bleaching of visual purple by rays 

 of different colour is very similar in shape to that which shows the 

 visibility of lights of low intensity, as shown in Fig. 277. The obvious 

 inference is that visual purple is the pigment concerned' with twilight 



40 



3-0 



2-0 



10 



00 



\\ 



JM<;. 1*77. Sli<>\\> UK- similarity between the curves representing the rate of bleach- 

 ing of visual purple by light of different wavelength and the luminosity curve 

 of twilight vision. 



vision. . When the visual purple in the retina has been bleached 

 by exposure to light, there follows a gradual reformation of purple 

 which is independent of nerve connections, but occurs only so 

 long as the stratum pigmenti is in contact with the rod epithelial layer. 

 If we suppose that the product formed by the bleaching of the visual 

 purple stimulates the rod apparatus, causing it to send impulses to the train, 

 we have at once obtained some idea of the mechanism used for night vision. 

 This we may briefly describe as follows : when light falls on the retina 

 certain rays, particularly those near the middle of the spectrum, are absorbed 

 by the visual purple. The pigment is bleached in proportion to 

 the light absorbed, forming a new product : this acts on the rods, causing 

 them to send impulses to the brain which continue so long as the light falls. 

 When the light stops the stimulating product is no longer formed, therefore 

 the stimulus to the rods ceases. 



It is stated that in diseases of the liver, in which there are bile salts circulating in 

 the blood, twilight vision is found to be impaired. This condition is ascribed to the 

 solubility of visual purple in bile salts, and it is thought that the removal of the [>'.u 

 ment from the retina prevents the rod apparatus from functioning. 



