REPOET OF THE SECRETABY 



127 



the use of a monochromator for obtaining narrower spectral regions 

 or purer colors. In this way more points could be secured in deter- 

 mining the response curve, and the amount of correction required for 

 shift of center of gravity minimized. The results of this second 

 experiment are shown in Figure 2. Points determined showing the 

 relative response as a function of wave length are indicated by solid 

 dots plotted on an arithmetic scale (inside frame). These points 

 have again been plotted as crosses on a logarithmic scale as indicated 

 outside the frame. The results of the earlier experiment are shown 

 as circles. 



iOOOOO 



10000 



100 



— 150 



— IZO 



— no 



— 100 



— fO 



— 80 



— 70 



— 60 



— SO 



— ij-O 



— 30 



— 2.0 



ffOOO SOOO 6000 7000 



Wave length 

 FiGURH 2. — rbototropism by monochromator method 

 — X — sensitivity on logarithmic scale (indicated outside of box) 

 — O — sensitivity on linear scale (indicated inside of box) 



Agreement between the two experiments is quite striking consider- 

 ing the rough nature of the earlier experiment. 



In the phototropic experiments the biological technique has been 

 developed by Doctor Johnston and the intensity relations determined 

 by Doctor McAlister. The demands upon physical technique were 

 so extreme that special vacuum thermocouples had to be developed 

 and the galvonometer deflection measured by means of a thermal 

 relay. 



It is interesting to note in this connection that Blaauw had secured 

 similar curves for phototropic response, measuring instead of relative 



