QUANTITATIVE DETERMINATION OF LIGHT REACTIONS 223 



The percentage differences in the two hghts used in this series of 

 experiments were 66f , 50, 33^, 25, 16|, S^ per cent, and as a con- 

 trol, equahty. The points on the basal arm of the apparatus 

 at which these percentage differences exist are: 66f per cent, 

 63.9 cm.; 50 per cent, 41.4 cm.; 33f per cent, 24.6 cm.; 25 per 

 cent, 17.4 cm.; 16f per cent, 11 cm.; 8| per cent, 5.4 cm. to the 

 right or left of the central point 0. The percentage differences 

 are in all cases expressed in terms of the stronger light. The 

 actual intensities of the beams are given in table 1 . 



With the apparatus set up for unequal opposed Hghts, negative 

 animals should deflect toward the weaker light, and the de- 

 flection should be greater as the difference between the lights is 

 increased. This deflection measured in degrees gives the desired 

 quantitative expression of the physiological efl'ects of the light. 



MATERIAL AND METHODS 



1. Cultures 



The raising of blowfly larvae, and the duration of the various 

 stages of their development under natural and artificial condi- 

 tions, has been dealt with by Herms ('07, '11). He also made 

 observations on the differences in behavior between certain of 



TABLE 1 



The intensities in candle meters of all the lights used in the series of experiments. 

 The four-gloiver intensity is slightly lower than would he expected because the 

 position of the glowers was such that there was a minimum of mutual heating at 

 this intensity. The 83i per cent intensities do not fit in with the series because 

 io obtain srich a large difference it was necessary to re-set the mirrors. 



