lO 



SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. 92 



There is a slight suggestion of two other maxima, one on each side 

 of the peak. It could not be determined from these data whether or 

 not these secondary maxima were real. Furthermore, certain condi- 

 tions existed during this preliminary experiment which make it impos- 

 sible to consider this sensitivity curve more than approximately cor- 

 rect. Although an attempt was made to burn the lamps at a constant 

 voltage, there was some fluctuation during the exposure of the seed- 



L \. - U ^ 



L_ — 1 1 1 1 1 — ^1 I I <=>— 



4tO0 4200 



■4^00 4500 4600 4700 



4900 5000 SIOO 5200 5300 



Fig. 3. — Phototropic sensitivity curve of preliminary experiment (continuous 

 line). The ordinates are relative sensitivity values, the abscissae, wave lengths 

 in angstroms, and the horizontal bars indicate the wave-length ranges of the 

 balance points. Circles indicate points obtained with filters combined with the 

 monochromator. Points more accurately determined are indicated by crosses 

 and connected by dash lines. 



lings and during the intensity measurements. Also, in some of the 

 work the standard lam]> as well as its filter cell was cooled by tap 

 water. This resulted in an accumulation of iron on the glass surfaces 

 during the time required for determining the balance points. These 

 uncontrolled factors undoubtedly modified to some extent the char- 

 acter of light transmitted. 



Because of the suggested secondary maximum on the longer-wave- 

 length side, three points on this side were again determined. This time 

 the lamps were connected to a battery of storage cells and the current 

 held more nearly constant. These three wave-length regions with the 



