158 REVERSED AND NON-REVERSED SPECTRA. 



If a plane grating is used, a weak lens L is attached to the rail R and moves 

 with it, so that its focus is in front of the ocular d (with cross-hairs) . In this 

 case 5 is a collimator. If a transmitting grating is examined, the collimator 

 5 (fig. 99), etc., are merely to be lowered, and the prism E is superfluous. It 

 need not even be removed. Naturally, it is in the interest of accuracy to have 

 all the standards like e and h as short as possible. 



Dx 

 Finally, in the equation X = , if D = io 6 d, the values d and R are usually 



of the same order (175 cm.) for gratings with about 15,000 lines to the inch. 

 In this case we may make the rail length R = d, whence 



Even in case of the concave grating, when ultimate precision is not aimed 

 at, some variation of the distance SS' = 2SE, nearly, is admissible without 

 destroying the definition. The carriage D with the prism E may be moved 

 fore and aft on the slides GG' until the focus at d is sharp. The values of x are 

 usually of the order of 100 to 125 cm., so that an accuracy of Angstrom units 

 is easily obtainable without special refinement. 



