SPECTRA OF PHOTOSYNTHETIC PIGMENTS 347 



D == ale = log J = log y, = log Y^rx' 



In this equation the optical density D bears a logarithmic relation to the 

 reciprocal of the transmitted light, h is the incident light, and I is the 

 transmitted light. D is directly proportional both to the length of the 

 light path within the solution and to the concentration of the absorbing 

 material. In heterogeneous systems, such as leaves or algae that scatter 

 as well as absorb, these relations no longer apply strictly. The non- 

 specular forward scattering of light, also termed "reflection," may be 

 of considerable significance. Furthermore, if a heterogeneous object 

 scatters light, the optical path length within it is no longer equal to the 

 thickness of the object, and it is not readily measurable. If, as in chloro- 

 plasts, the pigment is aggregated within grana, with clear spaces between 

 the grana, the concentration of pigment also becomes indeterminate for 

 optical purposes. The transmission of clear solutions is ordinarily meas- 

 ured in spectrophotometers, which pass a narrow beam of monochromatic 

 light through the solution and then to a photocell. Most spectropho- 

 tometers that do not depend upon the use of an integrating sphere, such 

 as that in the Hardy recording spectrophotometer, are not suitable for 

 measurements of Kght absorption by systems that scatter Ught, since the 

 photocells are ordinarily too far removed from the vessel to catch much 

 of the diffused light. Measurements of transmitted light may be made 

 fairly well by the use of a large photovoltaic cell or a piece of opal glass 

 placed directly behind the leaf or cell suspension. Such a method has 

 been used by Emerson and Lewis (1943) in measurements on Chlorella 

 to be discussed later in this chapter, and also by Chen (1951) on chloro- 

 plasts. This procedure does not, however, take into account the light 

 scattered and reflected from the front surface. Measurements of forward 

 and side scattering, as well as transmission of bacterial suspensions in 

 glass boxes of different sizes, are given by French (1937a). These data 

 have also been used to evaluate the deviations from Beer's law in scatter- 

 ing suspensions. An integrating sphere of the type ordinarily used in 

 making measurements of the total output of incandescent lamps is inher- 

 ently free of errors due to light scattering. This has been applied by 

 Seybold and Weissweiler (1942a,b) and by Rabideau et al. (1946) to 

 measurements of leaf absorption. The principles involved in sphere 

 measurements have been described in some detail by Kok (1948) . Figure 

 6-1 shows some of the different ways in which integrating spheres may be 

 used for this purpose. The elegantly simple arrangement of Haxo and 

 Bhnks (1950) requires a correction for the light reflected from the surface 

 of the photronic cell and absorbed by its second passage through the algae 

 (C. Yocum, personal communication, 1951). 



Rabinowitch (1951) reviews some means that have been used for 



