ULTRAVIOLET SPECTROSCOPIC TECHNIQUE 



151 



prism (Fig. 4-126), with the Young and ThoUon spht-prism arrangemont 

 (Kurtz, 1926), or with an adroit mirror arrangement recently described by 

 Makishima et al. (1951). In these instruments, the entrance and exit 

 slits (and hence the source and monochromatic image) are maintained 

 constant in position while the dispersing element(s) is rotated to vary 

 the emergent wave length. 



Grating Instruments. If a wave front of radiation is broken into a 

 number of narrow, parallel zones evenly spaced by appropriate distanc-es, 

 the waves propagating from each zone will interfere with those from all 

 other zones so as to produce a diffraction pattern. For any given wave 

 length, there will be some direction 

 or directions in which the waves from 

 each zone will all be in phase to pro- 

 duce a maximum of intensity. In 

 another direction, waves of another 

 wave length will be in phase to yield 

 a maximum of intensity, whereas the 

 waves of the first wave length will 

 largely cancel each other. A device 

 to thus disrupt a wave front is known 

 as a diffraction grating. By thus 

 deviating radiation of different wave 

 lengths into different angles, a grat- 

 ing can serve as a dispersing element. 



A grating may consist of a large 

 number of thin, parallel slits, in 

 which case it is a transmission grat- 

 ing, or of a similar number of thin, parallel reflecting strips, in which case 

 it is a reflection grating. 



If, in a direction of maximum intensity for a given wave length, the 

 waves from one slit (or strip) are exactly one wave length retarded or 

 advanced with respect to those from the two adjacent slits, this direction 

 is referred to as that of the first-order maximum. If the phase difference 

 between waves from two adjacent slits is just two wave lengths, the direc- 

 tion is that of the second-order maximum. In general, directions of 

 maximum intensity will occur whenever 



/)X = f/(sin a + sin i3) 



where n — order number (an integer) 



X = wave length 



d = spacing between slits. 

 (See Fig. 4-13). 



For a given wave length, the distribution of intensity in angle al)Out a 

 direction of maximum intensity, will d(^p(Mid ultimately (Hi iho ratio of th(^ 



Fic. 4-13. Diffraction by a plane trans- 

 mission grating. 



