6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. Sj 



of Ultra-\'iolet Light on a Unicellular Green Alga." ^ As will be noted 

 in comparing Plate 3, Figures i and 2, lines in the region from 

 2,500 A. to 3,000 A. appear in much their usual character, while the 

 very heavy lines shown between 3,000 A. and 3.200 A. are wholly 

 lacking. In order that such a biological spectrogram may be made to 

 yield a quantitative measure of lethal effect, one may resort to the 

 microphotometer methods which are in use for photographic pur- 

 poses. Again, the only essential difference is that one is dealing with 

 a plate of less uniformity and greater coarseness of structure in the 

 biological work. Consequently, a recording microphotometer of the 



I I I \ \ \ \ 



2500 zeeo aroo zeeo zsoo 3000 sioo 



Fig. 4. — Photometric record of algal spectrogram of mercury arc. 



Moll type may be used, provided it is equipped with unusually long 

 and wide slits, thus providing an integration over a considerable area. 



Figure 4 shows such a record by the microphotometer of the algal 

 plate. The lines are readily recognized although it will be observed 

 that the heavier ones have been so overexposed as to present a flat- 

 topped or truncated appearance. Such a thin plate effect may be 

 partially corrected by extrapolating the curves to a normal p3^ramidal 

 shape. This is, of course, only a first approximation, accurate work 

 requiring varied exposures so that one stays within a narrow range of 

 effective lethal action. 



Figure 5 shows a similar record by the microphotometer of the 

 photographic spectrogram, Plate 3, Figure 2. Although there is in 

 this case no evidence of a thin plate effect, the fact that the sensitivity 

 of the plate varies widely, not only for different wave lengths but 

 also for dift'erent intensities, is readily observed by a comparison of 

 this record with Figure 3. For convenience, the microphotometer 

 used has been equipped with extra large slits, special lenses being sub- 

 stituted for the microscopic type in standard use. Thermocouple and 

 second lens have been eliminated, a Weston photronic cell being sub- 



' Smithsonian Misc. Coll., vol. 87, no. 10, 1932. 



