﻿Rectification of the Spectrum. 



953 



the grating, I the incident ray. M and N the two opaque 

 mirrors symmetrically inclined relatively to the grating. 

 Rays R are refracted and rays D are diffracted. In fig. 8 



the spectra on the left of R are also available ; but they are 

 often complicated with the orders of spectra of the undeviated 

 ray before it has been reflected. To avoid these, one should 

 observe between the spectra of the non-reflected ray, or else 

 in the region of non-reflected spectra of the third or higher 

 orders, as these are too faint to interfere with the reflected 

 spectra. They often advantageously supply fixed lines in the 

 spectrum when arc light is used. 



To realize the condition of small angle, a collimator I and 

 the mirror N (micrometer movable parallel to itself) were 

 fixed on the edge of the spectrometer-plate diametrically 

 opposite to each other. The mirror M, the grating g g, and 

 telescope at R or D were revolvable around the axis of the 

 spectrometer. Grating and mirrors could be clamped. With 

 such an adjustment all values of 1, in fact, may be tested 

 between 10° and 80°, smaller and larger angles being- 

 excluded, because of the lateral dimensions of the apparatus 

 and mounting. 



The present method of interference is often exceedingly 

 convenient, inasmuch as the telescope at R is almost in 

 contact with the micrometer at N, which may therefore 

 be manipulated with ease by the observer. A typical form 

 of instrument now completed will be described elsewhere. 



The endeavour may now be made to compute B from the 



