2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 87 



ments. In a paper in process of publication Dr. F. S. Brackett and 

 Dr. E. D. McAlister will discuss more fully some of the physical prob- 

 lems that arise in connection with this work. 



RESULTS OF OTHER INVESTIGATORS 



The unicellular green algae, such as Chlorella, Pleiirococcus, Scene- 

 destnus, and Chlamydomonas , because of the similarity of their cells in 

 size, shape, and contents when in pure culture, can be grown fairly 

 homogeneously on a plate for exposure in the spectrograph. They thus 

 form excellent material for the study of the effect of ultra-violet light. 



As early as 1882 Engelmann placed green cells of Oedogouium, 

 Cladophora, and other algae in the spectrum of a microspectroscope to 

 observe the movement and accumulation of oxygen-loving bacteria in 

 those regions most favorable to assimilation. He used a constant gas 

 light and an incandescent electric light as sources of illumination. 

 Because of the great light intensities he was able to use a narrow slit 

 and so obtain a very pure spectrum. Ingenious as this method is, his 

 values are only approximate. Pringsheim ( 1886) using the same 

 general method found quite different values. 



Ward ( 1893) exposed plates of agar uniformly covered with bac- 

 teria to the spectrum and then observed the behavior of the illuminated 

 regions after incubation. He used the solar and " electric " spectra and 

 found that no detrimental action was perceptible in the infra-red, red, 

 orange, and yellow regions, but that all the bacteria were destroyed in 

 the blue and violet regions and far into the ultra-violet. 



Hertel ( 1905) was the first worker who made quantitative measure- 

 ments of the intensities of monochromatic light used for ultra-violet 

 radiation. His monochromator with its quartz prism and lenses was 

 similar to those now used in ultra-violet microscopy. He determined 

 the relative intensities of four lines of the ultra-violet part of the spec- 

 trum by means of a thermopile and he varied the intensity by regulat- 

 ing the amperage of the metallic arc. He found the region 2,800 A.' 

 to have a very destructive action on paramoecia and bacteria. 



In the past few years Cernovodeanu and Henri (1910), Browning 

 and Russ (1917), Mashimo (1919), Bang (1905), Bie (1889, 1905), 

 Bovie (1915), and a number of other workers have used the quartz 

 spectrograph for the study of the bactericidal action of light. Raybaud 

 (1909) made a spectrogram of three fungi. Hutchinson and Ashton 

 (1929), and Weinstein (1930) have studied the effect of monochro- 



^ A. ^ Angstrom units, i/u = i,(X!om/[i ^ 10,000 A. There are 100,000,000 A. 

 to the centimeter. 



