152 BIOLOGICAL EFFECTS OF RADIATION 



The slit characteristics of an instrument are extremely important in 

 all forms of monochromator. The second slit must coincide in geo- 

 metrical form with the image formed by the optical system of the first 

 slit, or else the normal advantages will not be secured. Since the range 

 of wave-lengths yielded by the instrument is critically dependent upon 

 the slit width, it must be accurately determined and readily reproducible. 

 In prism instruments, the off-axis deviation for a given wave-length is 

 considerably greater than that of the central point. Consequently, the 

 image formed of a straight first slit is curved thus demanding a curved 

 second slit. For many purposes, it is more convenient to work with a 

 straight second slit. It may therefore be desirable for the curvature 

 to be imposed upon the first slit. In order to secure satisfactory results, 

 the slits must be so adjusted that the image for a given wave-length falls 

 precisely upon the second slit. This requires adjustment for both focal 

 length and orientation. Since the amount of energy is dependent upon 

 the slit length, provided it can be illuminated uniformly, and upon the 

 width of the slits, the selection of suitable slit width and length is a 

 matter of great importance. The selection of suitable slit width is, of 

 course, dictated by the spectral purity required, while the slit length is a 

 matter of instrument design. 



With a suitably chosen monochromator placed in correct adjustment, 

 the proper illumination from a source must be secured. With some 

 sources it is feasible to bring the source so close to the slit, that the slit 

 is directly illuminated uniformly over its extent and width, and that the 

 solid angle subtended by the collimating lens is more than completely 

 filled from all portions of the slit. Such an arrangement is ideal, since 

 no added losses are incurred by the intervention of a condensing lens. 

 This method is, however, often precluded by the character of the source, 

 either because of its temperature, in that it cannot be brought sufficiently 

 close to the slit, or because it does not present an extended area of uniform 

 brightness. In such a case, it is customary to use a condensing lens, or 

 lenses, as shown by Ci and C2 in Fig. 6(a), between the source and the first 

 slit, which forms an image of the source upon the slit. In general, it 

 is then desirable that a uniform portion of the source entirely cover the 

 slit. Any convenient magnification or reduction of the image formed, 

 as compared to the source, may be used, provided the condensing lens 

 is of sufficient diameter and subtends a uniformly illuminated area. It 

 is necessary that the condensing lens be of sufficient diameter that the 

 solid angle of convergent radiation falling upon the slit be greater than 

 the solid angle subtended by the collimating lens. A safe rule is that 

 the diameter of illumination falling upon the plane of the collimating 

 lens be twice the diameter of the lens. This avoids any dangers in the 

 case of using narrow slits, from inhomogeneity of illumination due to 



