12 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 92 



Considerable difficulty was experienced in obtaining a satisfactory 

 balance point in the region of 4800 A. It was necessary to repeat 

 this part of the experiment several times. All other points gave con- 

 sistent data. It is possible that a slight shift of the seedling, one way 

 or the other from the center of the light beam, in this particular por- 

 tion of the spectrum was sufficient to account for the difficulty of 

 obtaining entirely satisfactory data. If this were true, then it would 

 indicate a considerable change in sensitivity over a range of only 100 

 angstroms at about wave length 4800 A. 



Table 4. — Data shoimng the Phototropic Effectiveness of Restricted Regions 



of the J^isible Spectrum. That for the Hg Line 435S A 



is taken as Unity 



» SG, Sextant green (1.94 mm); NC, Noviol " C " (4.15 mm); HRN, heat resistant Noviol 

 (3.04 mm). 



As mentioned above, the light source for this experiment was the 

 Mazda projection lamp. Most of the regions investigated were 

 approximately lOO angstroms wide. For the five points on the short- 

 wave-length side no filters were used. It is believed that the amount 

 of scattered light coming through the optical system of the mono- 

 chromator in this end of the spectrum did not greatly change the 

 character of the sensitivity curve. It is much more important to elimi- 

 nate the scattered light at the long-wave points. However, it seemed 

 advisable to determine one or two points on the short-wave-length 

 side by using the lines of the mercury arc that fall in this particular 

 region. A mercury arc in ]>yrex glass was set up in front of the 

 monochromator, and balance points were determined for lines 4047 A 

 and 4358 A. These points gave ratio values of 1.08 and 0.48 respec- 

 tively, and phototropic eiTectiveness values of 0.44 and i.oo. Both 

 points are indicated by crosses on the graph in figure 4. Because of 

 the purity of the 4358 A mercury line its value was taken as unity for 

 all the points given in table 4 and shown iii figure 4. 



