XIII. ACTION SPECTRA AND ABSORPTION SPECTRA 427 



intensities separately, so that the measurement is made at a single 

 reading. In the visual spectrophotometer the intensities of the two 

 beams are adjusted until they match to the eye, whereas in photoelec- 

 tric spectrophotometers, the difference in intensitj' of the two beams 

 is measured by the differential response of photocells. By appro- 

 priate devices the photographic plate may also be used photometri- 

 cally; two matched spectra are photographed — one with the ab- 

 sorbing material interposed, the other without — in such a way that 

 the plate blackening in the two cases can be compared directly without 

 correction for the characteristics of the plate. 



Methods of absorption spectroscopy have been developed for use 

 with the microscope, particularly the quartz microscope for the ultra- 

 violet. Here the microscope is focused on a given object in the cell, 

 and the absorption of a monochromatic beam measured by photo- 

 electric photometiy or in some other way. This method is limited 

 to objects whose diameter is about four times the wavelength of the 

 light employed or greater, and is a function of the angular aperture 

 {6,25). 



Materials in solution are usuallj^ contained in glass or quartz 

 absorption cells having flat parallel walls placed normal to the beam 

 in determining / ; and the same or a matched container filled with the 

 solvent is then used in determining h. In this way the ratio I^/I is 

 an index of the absorption of the dissolved substance, provided the 

 solution is reasonably dilute; factors such as absorption by the sol- 

 vent and reflection by the walls of the chamber cancel out, since they 

 are the same in both determinations. In measuring absorption by a 

 solid, say a glass color filter, /o is usually determined with no material 

 interposed. In this way one obtains the absorption with respect to 

 air, and may or may not take into consideration the reflection from 

 the surfaces, depending upon the purpose of the measurement. In 

 biophysics, various situations may arise in which it is desirable to 

 treat the question of reflection in different ways. 



2. Absorption Spectra of Substances in Living Systems 



Living systems provide a complex environment in which it may be 

 difficult to know the exact physical or chemical state of a given sub- 

 stance. The greater part of the living cell displays some degree of 

 organization, i.e., the molecules are at any given moment distrib- 

 uted in some characteristic pattern, which varies in its degree of 



