SPECTRAL PROPERTIES OF CELLULAR PIGMENTS 19 



substance changes from a solution to a solid. Let us turn to the more 

 complicated system of pigments inside living cells. The major compli- 

 cation in cells is the scattering of light by more or less colorless cellular 

 components. This scattering by colorless particles is caused by the 

 difference in the refractive index of the particles and that of the water 

 in which they are suspended. The refractive index changes slowly 

 with wavelength through the spectrum, the difference from that of 

 water being greater at shorter wavelengths. Very small particles, about 

 the size of a wavelength of light, scatter Hght in proportion to the 

 fourth power of the wavelength. Larger particles scatter with a smaller 

 value of the exponent, but blue is always scattered more than red. 



In addition to this scattering of particles which do not themselves 

 absorb light, there is a particular kind of scattering by colored parti- 

 cles. This effect is more important than has been widely realized. 

 Latimer and Rabinowitch (1956) made measurements of the intensity 

 of light scattered at right angles to an incident beam from a suspension 

 of Chlorella cells. Their results are shown by the middle curve of Fig. 

 3 (Latimer, 1957). The scattering is much greater on the long-wave- 

 length side of the pigments' absorption bands. This wavelength selec- 

 tive scattering is due to the sharp change of refractive index of the 

 pigment itself at wavelengths near an absorption band, the so-called 

 anomalous dispersion effect. 



A beam of light passing through a suspension of cells is attenuated 

 in two different ways: first by actual absorption and secondly by 

 scattering of both types. The scattering changes the direction in which 

 the beam emerges from the sample being measured, so that the light 

 may be only partially detected by the photocell as ordinarily placed 

 in a spectrophotometer. The top curve in Fig. 3 (Latimer, 1957) 

 shows a measurement of the attenuation of light by a suspension of 

 Chlorella measured with a recording Beckman spectrophotometer. 

 This instrument has its light receptor at a considerable distance from 

 the vessel containing the experimental material. Therefore, the instru- 

 ment records only the light coming from the suspension in a direction 

 nearly parallel to the incident light. In this narrow beam the attenua- 

 tion by scattering is far greater than the attenuation by actual absorp- 

 tion. The large relative contribution of scattering to the total attenua- 



