148 RADIATION niOLOGY 



elements, will transniil coinplotely only those wave lengths for which the 

 total rotation <it tlx- plane of polaiizalion is an integral nniltiple of 180°, 

 and will reject completely those wave lengths for which the total rotation 

 is an odd mnlti|)le of 90°. By cascading a few such elements of appro- 

 priately chosen secjuence of thickness, an over-all transmission band 

 width of 100-150 A at half-maximum transmission may be obtained. 

 The wa\e length of maximum transmission may be varied over a con- 

 siderable spectral region by a programmed rotation of the various polariz- 

 ing elements (Cambridge Thermionic Corp., 1952). 



This extension of the application of such filters to wave lengths as short 

 as 3000 A would be straightforward, since Polaroid will transmit well to 

 such wave lengths (Barer, 1949). Below 3000 A it would be necessary to 

 use prism polarizing elements of limited aperture. 



DISPERSING SYSTEMS 



Prism Instruments. Because of the variation of its refractive index 

 with wave length, a prism will deviate rays of different wave length 

 through dilTerent angles. If the angular spread of the radiation incident 

 on the prism is limited (by means of an entrance slit, or equivalent, and 

 collimating lens), the radiation emergent in any given direction will con- 

 tain a limited range of wave lengths. The emergent radiation may be 

 focused by a telescope lens to form a spectrum consisting of a continuous 

 series of images of the entrance slit in light of successively increasing 

 wave length. 



The width of the spectral band contained in any one image of the 

 entrance slit will depend on the angular divergence of the radiation inci- 

 dent on the prism, the rate of change of angular deviation produced 

 by the prism with wave length, and the physical breadth (in wave 

 lengths) of the beam emergent from the prism. The first (luantity is 

 determined by the entrance slit width and the collimator focal length. 

 The second quantity depends on the dispersive power (dn/d\) of the prism 

 material and the length of the base of the prism. The breadth of the 

 emergent beam determines the size of the diffraction disc to which it is 

 focused. 



By the use of an appropriately placed exit slit, any portion of the 

 emergent spectrum may be selected. The width of slit used can control 

 the spectral width of the radiation band transmitted, except that it is 

 inefficient to reduce the band width to less than the spread of wave lengths 

 contained in any single image of the entrance slit. 



In general, the band width, at half-maximum transmission, of radia- 

 tion emergent from a monochromator used with symmetrical entrance 

 and exit slits and similar collimator and telescope lenses, is given by the 

 formula 



AX = ^^ 



/•' X dd/d\ 



