REPORT OF THE SECRETARY 53 



devising a spectroscopic method wliich seemed feasible, he made 

 experiments vnth the so-called Christiansen filter, wliich are exceed- 

 ingl}^ promising. This apparatus consists of a parallel-walled, trans- 

 parent cell in which some organic liquid, as benzene is almost com- 

 pletely filled with granulated optical glass. At that wave length 

 where the index of refraction of the liquid is equal to the index of 

 refraction of the glass, the rays pass directly through, whereas those of 

 surrounding wave length are scattered to more or less wide angles. 

 It is easily possible with this apparatus to secure a nearly pure spec- 

 trum band not more than 200 angstroms wide. The band may be 

 moved from one part of the spectrum to another by changing the 

 temperature of the apparatus. Dr. McAlistcr, however, by mixing 

 two liquids, has arranged to alter the wave length of the transmitted 

 band by varying their relative concentration. With Mr. Hoover he 

 has developed an excellent method for illuminating with tliis source of 

 monochromatic light the wheat experiments and the experiments with 

 algae such as Dr. Meier is conducting. Quite sufficient intensities of 

 energy may be obtained by the devices proposed, and it is expected 

 soon to undertake the growth of wheat and of algae under selected 

 rays of nearly monochromatic light of suitable and controlled intensity. 



Dr. Meier made two series of check experiments on the propagation 

 of unicellular algae under 12 different varieties of light. These 

 experiments gave well-according results, but their interpretation as 

 regards wave-length data is not yet quite complete. 



Dr. Johnston is continuing the experiments on phototropism and 

 has attempted the growing of wheat under outdoor conditions with 

 different concentrations of carbon dioxide. Considerable difficulties 

 were encountered in tliis latter experiment, so that no definitive results 

 have yet been obtained, but it is believed that the way is now open 

 to more successful operations next season. 



Dr. Brackett has completed the installation of the rock-salt spec- 

 trograph for investigations of the absorption energy spectra of organic 

 substances in the extreme infrared, and has obtained some beautiful 

 preliminary photographic records of certain bands of absorption. He 

 has also completed the installation of a quartz spectrograph for the 

 study of absorption spectra through a wide range of wave lengths, and 

 has obtained beautiful records with it also, 



Mr. Clark has undertaken the preparation of suitable bimetallic 

 strips of cadmium and molybdenum for use in the construction of a 

 kampometer, that excessively sensitive radiation-measuring instru- 

 ment invented by Dr. Abbot and used by liim on Mount Wilson in 

 1932. 



Respectfully submitted. 



C. G. Abbot, Director. 



The Secretary, 



Smithsonian Institution. 



