ULTRAVIOLET SPECTROSCOPIC TECHNIQUE 147 



wave lengths is necessarily not in focus and is spread out more or less dif- 

 fusely. If the image of a small source is sharply masked at the focal 

 plane of the wave length desired, this wave lenj^th will be favored in the 

 radiation transmitted. If this process is repeated several times (the 

 same lens may be used in autocollimating schemes), quite narrow spectral 

 band widths may be obtained. Such a device is known as a focal isolation 

 filter. 



The selectivity of such a filter will depend on the dispersive power of the 

 lens material, on the number of lenses employed, and on the size of source 

 and angular aperture of transmitted radiation. Fluorite or quartz lenses 

 have been employed in this fashion to isolate spectrum lines in the vacuum 

 ultraviolet (Forbes et al., 1934; Duncan, 1940). 



Miscellaneous Filters. A simple ultraviolet filter which can be used in 

 well-collimated light to reject all wave lengths greater than an arbitrarily 

 chosen boundary, has been described by Regener (1936). A thin film of 

 paraffin oil is sandwiched between the long sides of two 90° quartz prisms. 

 Since the refractive index of the paraffin oil is less than that of the quartz, 

 total internal reflection can occur at the first oil-quartz interface; 

 owing to the greater refractive dispersion of the paraffin oil, there will be, 

 for any angle between the entrant beam and the oil-quartz interface, 

 some critical wave length above w^hich all wave lengths will undergo 

 internal reflection. This critical wave length can be varied by rotation 

 of the interface. Transmission is not complete at wave lengths imme- 

 diately less than the critical w^ave length, but increases rapidly with 

 decreasing wave lengths. 



Various types of light filters have found employment for special pur- 

 poses in the visible portion of the spectrum, and could undoubtedly be 

 adapted for use in the ultraviolet, but for one reason or another have not 

 been so used. Among these might be mentioned the polarization inter- 

 ference filter and the rotary dispersion filter. 



The former is based on the interference between two orthogonal com- 

 ponents of a beam of polarized light after passage through a birefringent 

 crystal; the retardation (in wave lengths) of the one component relative 

 to the other will depend on the thickness and birefringence of the crystal 

 and on the actual wave length, and hence varies wdth wave length, pro- 

 ducing maxima and minima of transmission throughout the spectrum, as 

 the interfering Avaves combine constructively or destructively (Billings, 

 1947; Evans, 1949a, b). Filters of this type, using cascaded birefringent 

 elements of appropriate sequence of thickness, have been made with a 

 band width of 1 A at half-maximum transmission (Billings et at., 1951). 

 Such filters are designed for use at a particular w^ave length. 



Rotary dispersion filters rely for their action on the variation in rotary 

 power of an opticall}^ active material, such as quartz, with wave length. 

 A piece of such material, placed between similarly oriented polarizing 



