ULTRAVIOLET SPECTROSCOPIC TECHNIQUE 149 



where W = width of exit sHt 



F = focal length of telescope lens 

 d6/d\ = angnlar dispersion of prism, 



or by 



W 

 AX = 



/„ X t dn/d\ 



where /„ = /-number of telescope lens 

 t = thickness of prism base 



dn/d\ = dispersive power of prism. 



The quantity of radiant energy transmitted through the dispersing 

 system depends intrinsically on the size of the entrance slit, the angular 

 aperture of the collimating lens, the height of the prism, and the size of 

 the exit slit. Increased energy may be obtained at the cost of spectral 

 purity by the use of wider entrance and exit slits. - 



In the ultraviolet region, 2000 4000 A, quartz (crystal or fused) is the 

 most commonly used prism material. Its dispersion increases rapidly 

 with decreasing wave length, which results in a corresponding increase in 

 spectroscopic resolving power in quartz prism instruments. Crystal 

 quartz of good quality may be used in prisms to wave lengths as short as 

 1850 A and in windows to about 1600 A (Powell, 1934b; Terrien, 1936; 

 Boyce, 1941; Gilles et al, 1949). 



Liquid-filled prisms may also be used in the ultraviolet (Forsythe, 1937, 

 pp. 88-89; Cannon and Rice, 1942), although these are temperature 

 sensitive and subject to such difficulties as convection currents due to 

 excess heating of the liquid near the entrant surface. Harrison (1934a) 

 has described some ingenious dispersing systems, employing water as the 

 dispersing element, in which convection is minimized by the use of the 

 upper surface of a water trough as the first surface of the dispersing 

 element. Such a device can be cheaply made in almost any desired size. 

 The dispersive power of water is within 50 per cent that of quartz. 



Below 2000 A, prisms of calcium or lithium fluoride may be used to 

 wave lengths of approximately 1300 and 1200 A, respectively (Powell, 

 1934a; Schneider, 1934, 1936, 1937; Kremers, 1940; Boyce, 1941; Stock- 

 barger, 1949). 



If uncorrected quartz lenses are used as collimator or telescope, these 

 must be refocused for different wave lengths. Achromatic quartz- 

 lithium fluoride pairs have been developed which are adequatelj^ cor- 

 rected throughout the ultraviolet and visible regions (Perry, 1932; Stock- 



^ An ingenious means for increasing the radiant flux transmitted through the dis- 

 persing system, while retaining high spectral purity, has been proposed by Shurcliff 

 (1949). In this proposal, which necessitates the use of two monochromators in 

 tandem, adroitly spaced, multiple entrance slits are employed, thus markedly increas- 

 ing the light input to the first monochromator. Appropriately placed secondary slits 

 in the spectrum plane between the two monochromators, and exit slits in the final 

 spectrum, then serve to exclude all save the desired wave-length region which is 

 obtained in high energy with essentially double monochromator purity. 



