PHOTIC REACTIONS OF TADPOLES 



267 



range. With the higher degrees of stimulation, the dependence 

 of the velocity of change upon the intensity is retained, but there 

 is a slight slowing down in this velocity. This condition is 

 further illustrated by the empirical formula for the curve pre- 

 sented early in this discussion (p. 250). The formula was found 

 to be as follows: 



94.48 



y = 



0.15 



+ 2.85 



in which y is the reaction-time in seconds, x, is the light inten- 

 sity in candle-meters, and 0.15, 2.85 and 94.48 are three con- 



TABLE 6 



Summary table, in seconds of average reaction-times, of products of reaction-times 



and light intensities, and of these values modified by subtractions 



{0.5 second and 0.7 second) for assumed secondary processes 



Reaction-time 



IXT 



Reaction-time — 0.5 sec. 

 IX T 



Reaction-time — 0.7 sec. 

 IXT 



LIGHT INTENSITY IN CANDLE-METEES 



173 

 121.1 



172.5 

 120.7 



172.3 

 120.7 



93.1 

 111.7 



92.6 

 111.1 



92.4 

 110.8 



24.1 

 120.5 



23.6 

 118 



23.4 

 117 



12.1 

 121 



11.6 

 116 



11.4 

 114 



127.5 



8.0 

 120 



7.8 

 117 



20 



7.6 

 152 



7.1 

 142 



6.9 

 138 



30 



6.1 



183 



5.6 

 168 



5.4 

 162 



60 



2.9 

 174 



2.4 

 144 



2.2 

 132 



1 .7 

 340 



1.2 

 240 



1.0 

 200 



500 



0.76 

 380 



0.26 

 130 



0.06 

 30 



stants. This formula satisfies well the actual data obtained. 

 The constant a of the original formula (p. 250) is equal to 0.15 in 

 the empirical representation and means that with the lower 

 intensities of light the Bunsen-Roscoe law does not hold true 

 for all values of x, but that there is a certain minimum intensity 

 of light beyond which the light produces no apparent effect 

 upon the organism. This was demonstrated experimentally. 

 The constant b ( = 2.85) denotes that with the higher intensities 

 and as the degree of illumination approaches infinity, a point is 

 reached where the reaction-times are the same regardless of the 

 intensity of illumination. This point in the reaction-time of 

 Rana clamitans, however, is not reached abruptly, but gradually 

 and with a definite constancy. 



