488 S. O. MAST 



used is known.2 It was consequently not necessary to make meas- 

 urements of the absolute illumination used, and this constitutes 

 one of the most advantageous features of the method em- 

 ployed, for it simphfies greatly the work involved. We shall, 

 however, give sufficient data in the following paragraphs and in 

 connection with the description of the individual experiments to 

 make it possible to reduce the readings as presented in the fol- 

 lowing pages to terms of absolute intensity in case anyone should 

 desire to do so. 



In all but a few preliminary experiments, which were made 

 before August 1 the current used was 5.75 amperes with only very 

 shght variation. With this current and with the Lummer- 

 Brodhun sector set at 100, the candle power in the white beam 

 was 40.6. In all of the experiments except those on earth- 

 worms and blowfly larvae the source of fight in the white beam 

 was 50 cm. from the center of the observation aquarium. The 

 illumination in this beam with the sector set at 100 was therefore 

 approximately 1'62.4 meter-candles at the center of the field of 

 observation. It was actually somewhat less owing to reflection 

 and absorption. To calculate the illumination with the sector 

 set at any other point it is only necessary to read the number 

 on the scale opposite the point in question, divide it by 100 and 

 multiply the quotient by 162.4, i.e., by the illumination in meter- 

 candles with the sector at 100. In this way the illumination 

 can readily be calculated for any of the sector readings in the 

 following tables except those which refer to earthworm ; and 

 blowfly larvae. In the experiments on these two forms the 

 distance between the center of the field of observation and the 

 source of fight in the white beam was 100 cm. and the fight was 

 also reduced by screens so that with the sector set at 100 the 

 candle-power was in some of the experiments 21.4, in others only 

 7.4. These numbers must therefore be used- in place of the 

 preceding in calculating the illumination. In these experi- 

 ments the distance .between the ocular slit of the spectrometer 

 and the center of the field of observation was 53 cm. and both 

 beams of light were 6 cm. wide (see p. 517). 



*See figures 3 and 4, and tables 2 to 15. 



