104 p. LATIMER AND E. I. RABINOWITCH 



of refraction. According to the classical theory of dispersion, the 

 index of refraction of an absorbing medium should reach a maximum 

 on the low-frequency side of an absorption band. This is in fact where 

 scattering maxima have been observed by us. 



A similar spectral selectivity has been reported recently by Goed- 

 heer (4) and by Menke and Menke (5) in the double refraction of 

 chloroplasts from Mougeotia. The quantity which was measured i]i 

 those cases is the difference between the indi(;es of refraction of the 

 material for light beams polarized in two different planes; scattering 

 depejids on the average index of refraction. 



The scattering of protons by atomic nuclei shows the same type of 

 selectivity for proton energies in the neighborhood of the resonance 

 levels (6). 



Further study of selective scattering may supply information about 

 the packing and arrangement of pigment molecules in biological 

 systems, and may even reveal the presence of pigments which are not 

 easily found by other techniques. As an example, it is seen in Fig. 2 

 that the selective scattering produced by the carotenoids in Chlorclla is 

 much more conspicuous than their contribution to the absorption 

 spectra. The latter is so effectively obscured by chlorophyll absorption 

 that the very existence of carotenoids as such in green cells has been 

 doubted (7). 



We wish to thank Professor James Franck and Professor Robert 

 Emerson for helpful suggestions and Ruth V. Chalmers for growing 

 the algal cells. 



TRANSMISSION CHANGES IN ILLUMINATED CELL SUSPENSIONS 



We have heard several reports describing small changes in the trans- 

 mission of collimated beams by cell suspensions. It may be worth 

 while to draw attention to the fact that changes in scattering, as well 

 as changes in true absorption, could lead to observations of this type. 



The figures in the present paper show that, in some regions, light 

 scattered by cells varies shai-ply with wavelength. The physical struc- 

 ture of a cell, in which the selective scatteruig originates, may ])e 

 altered by exposure to light, or by chemical agents. In either case, 

 changes in selective scattering could result and lead to selective 

 changes in transmission. Some investigators have found variations in 

 transmission after changes in temperature (Witt), or after the addi- 



