500 Absorption Spectrophotometry /26 : 5 



where A is a proportionality constant. Thus, the output of the a-c 

 amplifier is proportional to the optical density. 



If the signal is very small, the losses in the logarithmic attenuator are 

 excessive. In this case, it may be omitted, since 



if 



(A - h) < h 



Under these conditions, the optical density will be proportional to 

 I x — I 2 . If a 10 per cent error in optical density can be tolerated, the 

 approximation may be used, provided that the total change of optical 

 density does not exceed 0.05. 



Motor Number 2 in Figure 12 controls both the wavelength dial and 

 the recorder. This makes possible a recording of the entire absorption 

 spectrum. The filter shown in Figure 12 helps to remove chopping 

 noise and high-frequency photomultiplier noise. 



This instrument has been described as a specific example of an 

 absorption spectrophotometer which is more complex than a single- 

 beam one. Numerous commercial as well as laboratory-constructed 

 models of spectrophotometers — single-beam, split-beam, and dual- 

 beam — have played an essential role in research in all of the biological 

 sciences. 



REFERENCES 



1. Brode, W. R., Chemical Spectroscopy 2nd ed. (New York: John Wiley & 

 Sons, Inc., 1943). 



2. Harrison, G. R., R. G. Lord, and J. R. Loofbourow, Practical Spectroscopy 

 (Englewood Cliffs, N.J.: Prentice-Hall, Inc., 1948). 



3. Oster, Gerald, and A. W. Pollister, eds., Physical Techniques in Biological 

 Research. Vol. I. Optical Techniques (New York: Academic Press, Inc., 

 1955). Particularly Chapters 1 through 7. 



4. Blair, W. L., A Split Beam Spectrophotometer Thesis, The Pennsylvania 

 State University (1958). 



5. Bauman, R. P., Consulting ed., Biological Applications of Infrared Spectros- 

 copy (Monograph) Ann. New York Acad. Sc. 69: 1-254 (1957). 



