346 RADIATION BIOLOGY 



may be given as percentage of absorption, optical density, or the loga- 

 rithm of the optical density. These various scales have, for certain pur- 

 poses, different degrees of usefulness. Any comprehensive intercompari- 

 son of data requires, however, that the scales be identical. We have 

 therefore replotted all the curves presented in this paper on the same 

 wavelength. Even with uniformity as an objective, it has not been practi- 

 cal to use the same scale throughout this article. For most purposes 

 optical-density scales are preferable, but for comparison with action 

 spectra per cent absorption curves are generally used. Many curves 

 originally given as percentage of transmission or as logarithm of density 

 have been converted to optical density, and in most cases the optical 

 density of live material at the position of the chlorophyll peak in the 

 neighborhood of 678 m/z has been adjusted to 0.6, thus making possible 

 a direct comparison between many different curves by simple tracing. 

 These transformations have been carried out by use of a graphical comput- 

 ing machine (French et al., 1954). Many absorption spectra are published 

 with much too short a wave-length scale and an exaggerated height. 

 These may have a dramatic appearance but are nearly impossible to 

 use in finding the absorption coefficient for a particular wave length. 



ABSORPTION SPECTRA 



MEASUREMENT OF ABSORPTION SPECTRA IN SCATTERING MEDIA 



The measurement of true pigment absorption in leaves or suspensions 

 of algae which, in addition to absorbing, also scatter large amounts of 

 light in all directions is indeed difficult. Many investigators have taken 

 up the problem of making such measurements in leaves. We have here 

 omitted reference to all work without scattering corrections, work done 

 with broad spectral regions, or data including too few wave lengths to 

 give complete spectra. The papers by Seybold and Weissweiler (1942a, b) 

 are much more extensive than any of the previous work and review the 

 absorption-spectrum measurements of many other workers. Confirma- 

 tory data and a small extension of the wave-length range toward the near 

 infrared are given by Rabideau et al. (1946). More recent absorption 

 and reflection data are given by Moss and Loomis (1952). Specific work 

 on the absorption of particular algae will be referred to later in connection 

 with their action spectra. 



Taking the absorption of light by a clear homogeneous solution after 

 correction for specular reflection of light at the surface of the vessel, we 

 have the relation A -f T = 1 , where the incident light is set equal to 1 ; 

 A and T are, respectively, the absorption and transmission of the solu- 

 tion concerned. The transmission varies with the thickness I of the layer, 

 the concentration c of the absorbing material, and the absorption coef- 

 ficient a according to Beer's law: 



