PHOTIC REACTIONS OF TADPOLES 251 



Henri and Henri ('12) carried out a series of studies on the 

 effect of ultraviolet rays upon cyclops. From their data it appears 

 that in producing a reflex, the intensity-reaction-time products 

 do not give a constant, but that the hyperbolic curve obtained 

 can be expressed by the formula: 



y = - + k. 

 X 



Hecht ('18, '19, '20), in studying the photosensitivity of Ciona 

 and Mya to different intensities of light emitted from an electric 

 bulb, reports that the products of the sensitization periods and 

 corresponding intensities agree well with each other for all in- 

 tensities. 



The Bunsen-Roscoe ( '62) law was applied originally to the influ- 

 ence of light on silver chloride. In the responses of tadpoles, 

 there is a striking parallelism between that which has been demon- 

 strated to be true for a purely chemical phenomenon 'in vitro' 

 and the changes occurring ' in vivo, ' namely, in the receptors of 

 the tadpoles. -^ 



The Bunsen-Roscoe law may be expressed : 



{l)RT XI =K, 



where RT \s, the time of action, /, the intensity of the light, and 

 K, the constant effect produced. Equation (1) may be put in the 

 following forms : 



(2) RT =^j 



(3) log i^r = log K - log 7, 



(4) log RT = - log I. 



In the last form of the equation (4) the logarithm of the 

 reaction-time is plotted as function of the logarithm of the 

 corresponding intensity, and if the data in table 2 are plotted in 

 accordance with this form, the points give an approximately 

 straight line (B), thus indicating that in the production of a 

 constant effect the velocity of change in the receptors proceeds at 

 a uniform rate. With the gradual increase in the intensities 

 of illumination there is an acceleration in the rate of the changes 

 in the sense organs. 



