104 Bar us — Use of the Grating in Interferometry. 



liable to be irregular. Inclining the collimator moves the 

 spectrnm across the fringes vertically, while inclination of the 

 telescope moves both equally. In this way the centers may 

 often be found. With strong tipping, however, the figure 

 becomes distorted or open above and below, as would be 

 expected. If the liglit is intensified for projection or naked- 

 eye work, there is also distortion. 



It is not necessai-y and apparently of little advantage to 

 have the refiections from the mirrors 31 and N occur at normal 

 incidence. In fact the patch of white light on the grating 

 surface and the return patch of spectrum may be over an inch 



Fig. 1. 



apart. Inasmuch as the spectra are rigorously coincident, it 

 follows that (apart from the refraction of the thick plate glass) 

 the grating mnst be symmetrical with respect to the mirrors ; 

 i. e., intersect the angle between tliem. Very little difference 

 of size or shape is observable between extremes of such adjust- 

 ment. Thus I rotated the grating about 10° (larger angles 

 being unavailable), without appreciable effect, after one of the 

 mirrors had been correspondingly adjusted. 



On examination of the effective air distances of the mirrors 

 M and N from the grating, it is found that three positions are 

 favorable to interference ; in the first case (case A), M is 

 farther away than N from the corresponding faces of the 

 glass plate; in the second (case B), they are about equidistant; 

 finally in the third (case C), iT is farther away, symmetrically 

 with the first adjustment. For each case the admissible play 



