NO. 10 ULTRA-VIOLET LIGHT ON GREEN ALGA MEIER 5 



RESULTS 



Decolorized regions appeared where the plate was exposed to wave 

 lengths 2,536, 2,652, 2,699, 2,753. 2,804, 2,894, 2,967 and 3,022 A. 

 Those algae exposed to wave lengths 3,130, 3,341, 3,650 A. were 

 unharmed and the cells were filled with green chlorophyll. Further- 

 more, it may be noted that wave lengths 3,130 and 3,650 A. are more 

 intense by actual thermocouple measurements, as shown in Table i. 



McAlister, by the use of the double monochromator and extremely 

 sensitive thermocouples, has accurately measured the energy distribu- 

 tion in the mercury arc in the ultra-violet region between 2,000 and 

 4,000 A. In Plate i the first algal spectrogram obtained in this experi- 

 ment is superimposed on McAlister's record of the mercury-arc spec- 

 trum. The ordinates given here in centimeters of galvanometer deflec- 

 tion are proportional to the intensities measured with the double mono- 

 chromator. For quantitative comparison these intensities have been 

 corrected for the relatively lower transmission of the fused quartz 

 system of the spectrograph used in this experiment. 



Table i gives the intensities of the lines used and the computation 

 of the relative lethal sensitivity to each line. Duplicate natural-color 

 plates were made of the first algal spectrogram which was exposed for 

 21 minutes over the entire length of the slit. Black and white copies 

 of these color plates were then made, and a densitometer record was 

 determined on a Moll recording microphotometer. The curves of the 

 density of the silver in the photographic emulsion correspond here to 

 the algal density. A photometer record was also made of the composite 

 superimposition of the two negatives of the color plates, the photo- 

 graph of which is shown in Plate 2, Figure 2. The areas under the 

 photometer curves corresponding to the intensity of the lethal effect 

 were measured with a planimeter. In cases where the plates were 

 obviously so thin that the densitometer record appears as a truncated 

 pyramid, extrapolation to a normal curve has been made in order to 

 correct as far as possible for this source of error. The probability is 

 that the stronger lines are still undercorrected. The average of the 

 areas of these three densitometer records gives the best available data 

 for the first traverse of the color plate. 



A second traverse, that is, a densitometer record across another 

 region of the plate, was made in order to obtain a better representative 

 determination and so minimize the inhomogeneity of this plate. The 

 uniformity in the second traverse is such that equal weight has been 

 given to it with the average area determinations of the first traverse. 



Mean area (A) is the average of the areas from the first and second 

 traverses. These areas should give a reasonably good measurement 



