SELIG HECHT 501 



Not content with the mere statement of such facts, the investiga- 

 tors beginning with Piper have presented their data in terms of sen- 

 sitivity or Empfindlichkeit. As used in this connection these two 

 words signify some multiple of the reciprocal of the minimum inten- 

 sity. The actual units of sensitivity vary. Piper uses a million 

 times the reciprocal, whereas Nagel considers Empfindlichkeit as the 

 simple reciprocal of the minimum intensity. It is apparent, how- 

 ever, that the two are essentially the same thing. The data shown 

 in Fig. 1 are given in terms of Piper's imits of sensitivity. 



It is here that we meet the first difficulty. Sensitivity as defined 

 in this way possesses no meaning other than that inherent in the 

 original fact of the minimum intensity. It is true, speaking in a gen- 

 eral way, that the irritability of the eye increases as the minimum 

 intensity necessary to stimulate it decreases. But we must not be 

 deceived by so seducive a word as sensitivity, even when it is accom- 

 panied by certain figures purporting to represent the number of 

 units of this condition. It is so easy to forget this, and to apply the 

 term as a quantitative estimate of the condition inside the eye {Emp- 

 findlichkeit der Netzhaut) instead of remembering that it applies 

 merely to the condition of the outside light. Empfindlichkeit includes 

 nothing more than what is implied in the minimum intensity neces- 

 sary to elicit a visual effect in the eye. 



However, even as a statement of the changes in the external light, 

 the use of Empfindlichkeit or sensitivity is attended with the danger 

 that it distorts the actual course of retinal adaptation. As a matter 

 of fact the shape of the curve in Fig. 1 and its division into three 

 phases represent neither the properties of the retina nor those of the 

 light. It does represent a certain property of numbers. The whole 

 thing depends on the simple fact that, as a number decreases, its 

 reciprocal increases in a curious way. For example 



is the equation of a straight line. However 



1 



y=z- 



' X 



that is, using the reciprocal of x is not the equation of a straight 

 line at all, but that of an equilateral hyperbola. 



