SECTION 111 



THE SPECTRUM AS SEEN BY THE DARK-ADAPTED 



(SCOTOPIC) EYE 



CHAPTER I 



ADAPTATION OR TEMPORAL INDUCTION 



When we pass suddenly from bright sunlight into a dimly lighted 

 room we can see nothing for a time until we become " accustomed to 

 the darkness." When we pass from the dark into bright light vision 

 is also difficult and may be painful. We therefore infer that the sensi- 

 bility of the retina becomes increased at low illuminations. This 

 automatic process of levelling the sensibility of the retina to the require- 

 ments of the moment is called dark or light " adaptation " (Aubert) 

 or " temporal induction." 



Dark adaptation is a relatively slow process. It is characterised 

 by a rise in the sensitiveness of the retina to light, which is slow during 

 the first ten minutes of exclusion of light from the eyes, rapid during 

 the following twenty or thirty minutes, and again slow or almost 

 negligible after that period. The general character of the curve of 

 retinal sensibility is the same in all cases, but there are marked individual 

 variations in the rapidity and amount of the rise, thus explaining the 

 fact that some people see very much better in a dull light than others, 

 though variations in the size of the pupils and other factors (vide infra] 

 are not without importance in this respect. In night-blind people 

 there may be only a very slow rise, the ultimate sensibility after an 

 hour being near the normal limit. In severe cases there is very little 

 rise even after several hours. 



Such adaptation is normal in the colour-blind, even the totally colour- 

 blind 1 . Strychnin and brucin cause increase in the amount and rapidity 



1 Tschermak, Ergebnisse d. Physiol. i. 1. 700, 1902. 



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