SPECTRAL PROPERTIES OF CELLULAR PIGMENTS 23 



have been presented by Jacquez and Kuppenheim (1955). Several 

 arrangements of integrating spheres as used for photosynthetic organ- 

 isms are discussed by French and Young ( 1956) . The sphere does not, 

 however, correct the measurements for the variation of the average 

 path length of the light within the sample. Owing to scattering, this 

 path length varies with wavelength and so may distort the shape of the 

 absorption spectrum. 



While a sphere is probably the best method of finding out how much 

 light is actually absorbed by a suspension for such purposes as 

 quantum yield measurements, there are simple procedures that give 

 just as good results for many purposes. A large spherical mirror with 

 a monochromator and photocell has been used to measure diffuse 

 reflection spectra of leaves. Reversing the photocell and placing the 

 mirror on the other side of the sample from the light beam makes a 

 reasonably good absorbance spectrophotometer with a gathering angle 

 of about 90°. Another good procedure for laboratory setups used by 

 Emerson and Lewis (1943) and by Haxo and Blinks (1950) is to 

 put the sample in direct contact with a barrier layer photocell of large 

 area. 



Methods summarized by Shibata (1956) depend on the diffusing 

 properties of opal glass. The great value of Shibata's methods is that 

 they may be applied easily with common recording spectropho- 

 tometers, and the cost of the extra equipment is negligible. The 

 principle of the opal glass method for measuring absorption spectra 

 of suspensions and other translucent biological materials is illustrated 

 by Shibata, Benson, and Calvin (1954). The opal glass mixes the 

 light that comes out of the suspension at various angles. Therefore 

 the light reaching the photocell is a more or less representative sample 

 of all the light from the vessel, regardless of its angular distribution. A 

 simple holder for measuring leaves or other translucent sheets of 

 material with opal glass in the recording Beckman spectrophotometer 

 is described by Smith, Shibata, and Hart (1957). 



An opal glass device has been developed by Shibata (1957a) for 

 the absolute measurement of reflection spectra with a recording 

 spectrophotomster. Details of the various ways in which opal glass 

 may be used to measure absorption and reflection, and the absorption 

 spectra corrected for reflection as needed for quantum yisld experi- 



