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GRAPHIC CORRELATION OF RADIATION AND 

 BIOLOGICAL DATA 



By F. S. BRACKETT 

 Chief, Division of Radiation and Organisms, Smithsonian Institution 



In discussions of the relation of radiation to biological phenomena 

 one frequently wishes to correlate transmission curves and the char- 

 acteristics of common sources of light with the response curves of the 

 biological phenomena. Although the facts involved are for the most 

 part well known, they are scattered through the literature in such a 

 way that it is difficult to form a clear picture of their interrelation 

 without gathering this material together graphically. In order to meet 

 this need the composite graph shown in Figure i has been developed. 

 The accompanying explanation indicates the significance of each 

 curve, and the bibliography at the end of the paper will enable anyone 

 who wishes more detailed data to go immediately to the original 

 sources. 



As water is the chief constituent of most living matter, its trans- 

 mission characteristics set definite limits for other than surface efifects 

 of radiation. It is perhaps significant that radiation therapy has found 

 its effective wave-length regions in those ranges where transition takes 

 place from negligible transmission to relatively great transmission. 

 Such a region exists in X rays from lA to shorter wave lengths, and 

 again in the ultra-violet for wave lengths immediately longer than 

 .18/A. Another region which has as yet been little studied occurs in 

 the near infra-red for wave lengths shorter than 1.4/i. The trans- 

 mission characteristics of water may perhaps most readily be indi- 

 cated by plotting the absorption coefficients, that is k in the expression 

 I — 1^0-^-^, as a function of wave length or frequency over the regions 

 of interest. The full line curve a iji the upper section indicates these 

 values for the range from io/a to .i/t in wave length as indicated at 

 the bottom of the graph, or .1 to 10, X 10* wave number (waves per 

 cm, i. e., proportional to frequency) as indicated at the top. The values 

 of the absorption coefficients are shown at the left outside the frame. 

 Another convenient method is to indicate for each wave length or 

 frequency the thickness of water which will reduce the light to one-half 



Smithsonian Miscellaneous Collections, Vol. 87, No. 8. 



