202 RADIATION BIOLOGY 



tion and temperature control. Consequently their use has been limited 

 principally to astronomical investigations (Billings, 1947). Such filters 

 can be tuned electrically with Kerr-cell birefringent components and the 

 band pass shifted rapidly through a limited portion of the spectrum. 



SPECTROSCOPIC METHODS 



The most versatile instrument for the isolation of narrow spectral 

 regions is the spectroscope, which is any device capable of producing a 

 spectrum. It usually consists of three basic components: a collimator 

 comprised of a slit and lens or mirror for producing parallel rays, a dis- 

 persing element such as a prism or grating, and a focusing system for 

 producing spectral images of the entrance slit. If the spectrum is imaged 

 on a photographic emulsion or other surface, it becomes a spectrograph ; 

 if a second slit is interposed in the spectrum, thus isolating a single narrow 

 band of wave lengths, it becomes a monochromator. Since the design 

 and use of such instruments have been extensively covered by several 

 monographs and papers on spectroscopy (French et at., 1947; Harrison 

 et al., 1948; Sawyer, 1944) and spectrophotometry (Erode, 1943; Gary 

 and Beckman, 1941 ; Lothian, 1949; Mellon, 1950; Walsh, 1952; Wilhams, 

 1948; Zscheile, 1947), only a few of the more important considerations 

 pertaining to the selection and use of irradiation spectroscopes will be 

 discussed. 



Dispersing Elements. The dispersing element of the spectroscope 

 bends the incoming rays through various angles, depending upon the 

 wave length. This is quantitatively expressed as the angular dispersion 

 d6/d\, where dd is the increment of angular deviation in radians corre- 

 sponding to the wave-length interval d\ (Fig. 3-22). A more directly 

 useful quantity is the linear dispersion or the linear separation of small 

 increments of wave length in the focal curve of the spectrum. It is 

 principally a function of the angular dispersion, the focal length of the 

 focusing lens or mirror, and the inclination of the focal curve. The 

 resolving power of a spectroscope is defined as \/d\ and is the wave- 

 length interval between two adjacent lines which can just be resolved 

 with the narrowest slits. 



1. Dispersing prism. The dispersing prism of the spectroscope is a 

 refracting element in which use is made of the variations in refractive 

 index with w£ive length. Thus rays of different wave lengths are bent 

 through different angles in passing through a prism. For minimum devi- 

 ation in a simple prism (Fig. 3-22a) the angular dispersion is related 

 approximately to the change in refractive index with wave length dn/dX 

 as follows: 



dd dn 2 tan i . 



d\ dX n 



