210 RADIATION BIOLOGY 



astigmatism and spherical aberration increase rapidly with decrease in 

 focal length. As with a camera lens, the depth of focus increases as the 

 aperture ratio decreases, and focusing becomes correspondingly less criti- 

 cal. The cost of short-focal-length lenses and mirrors of adequate quality 

 for a given instrument is usually much greater than that of long-focal- 

 length optics of the same diameter. Another factor tending to reduce 

 the cost of small-aperture-ratio optics is the availability of long-focal- 

 length astronomical parabolic mirrors of excellent quality in // numbers 

 of 8-12. 



2. Comparison of prisms and gratings. The angular dispersion of the 

 prism may be very high in the shorter wave lengths, but it decreases 

 rapidly with increase in wave length. However, the transmission of a 

 prism is high throughout most of its useful range. By contrast, the dis- 

 persion of the grating is nearly constant and may greatly exceed that of 

 a quartz or glass prism in the longer wave lengths. The effective trans- 

 mission is relatively low for an echelette grating in all but the spectral 

 region of the "blaze," where the intensity is maximal. The spectral 

 position of the blaze is determined by the groove angle. 



The slit images or lines produced by a grating are less curved than 

 those produced by a prism. This is due mainly to the smaller angle of 

 incidence usually employed with the grating. Thus, with a grating, 

 longer slits may be used without having to resort to curved entrance 

 slits to obtain straight-line images. The relatively uniform dispersion of 

 gratings makes it practical to use interchangeable fixed slits instead of 

 continuously variable slits, as required by the varying dispersion of' 

 prisms. This is an important advantage where water-cooled entrance 

 slits are required. Gratings have had the reputation of scattering a rela- 

 tively large proportion of undispersed energy. The aluminized replica 

 grating is a great improvement over the speculum metal grating in 

 this regard. A well-designed plane-reflection-grating spectroscope with 

 reflecting optics approaches comparable prism instruments as to freedom 

 from stray flux (Donaldson, 1952). 



Quartz and glass prisms are rugged, easily cleaned, and relatively 

 permanent. Gratings are fragile, and the aluminum is damaged by cor- 

 rosive atmospheres and cannot be readily cleaned. For the same effective 

 area, the plane replica grating is much less expensive than high-quality 

 prisms. Conseciuently, for the same cost, a larger aperture area can be 

 obtained in a grating than in a prism (Harrison et at., 1948). 



Harrison et al. (1948) and Strong (1949) have reported detailed per- 

 formance data on prism and grating spectroscopes. For the biological 

 irradiation spectroscope or single monochromator in the ultraviolet and 

 visible, the plane reflection grating of large aperture area appears to offer 

 many advantages over the prism. For the double monochromator, where 

 the combined power transmitted is proportional to T", prism instruments 



