XIII. ACTION SPECTRA AND ABSORPTION SPECTRA 



438 



Sources of radiation are of two types, presenting somewhat dif- 

 ferent problems with regard to the isolation of monochromatic radia- 

 tion. ''Line" sources, such as, for example, the mercury arc, emit 

 only a limited number of wavelengths, which appear in the spectrum 

 as discrete lines. The wavelengths correspond to the sizes of quanta 

 emitted when electrons jump from higher to lower energy levels in 

 the atom, and since the levels are characteristic of the emitting atom, 



1.0 



• Absorption of rhodopsin 



Photosensitivity of rhodopsin 

 o Sensitivity of aphakic eye 



300 



350 



400 



550 



600 



650 



450 500 

 WAVELENGTH, mu 



Fig. 5. Comparison of absorption spectrum of rhodopsin, spectral sensitivity 

 of a solution of rhodopsin, and spectral sensitivity of scotopic vision in aphakic 

 (lensless) subjects (^5). 



the wavelengths of the lines emitted by a given kind of arc are al- 

 ways the same, although their relative intensities may vary. With 

 such sources it may be a relatively easy matter to isolate virtually 

 monochromatic radiation by means of a monochromator or filters. 

 The difficulty with such sources is that they provide only a limited 

 number of wavelengths, often too widely separated to furnish ade- 

 (luate detail for an action spectrum. Actually, the only source of 

 this type developed to a sufficient state of dependability for many 

 biological studies is the mercury arc. This arc provides a sufficient 



