Interference with Reflecting Gratings. 119 



The present method is the converse of this, since the 

 gratings and the opaque mirrors change places. Parallel 

 rays from L strike the plate of glass M and the component 



Fiff. 1. 



rays reach identical reflecting gratings Gr m and G„, placed 

 symmetrically with respect to M at' an angle i to the E and 

 L directions. The undeviated rays pass off eccentrically at 

 R and are not seen in the telescope at E. They may, 

 however, be seen in an auxiliary telescope pointed in the 

 line R, and they then facilitate the adjustments. Rays 

 diffracted at the angle 2?', however, are respectively trans- 

 mitted and reflected by M and interfere in the telescope in 

 the line E. Similarly rays diffracted at an angle & >i 

 interfere in the line D. 



To make the adjustment it is sufficient to bring the 

 Fraunhofer lines in the two spectra seen at E into complete 

 coincidence, horizontally and vertically. Coincidence of slit 

 images at R (at least vertically) aids in the same result. It 

 is also necessary that the rulings on Gr m and G n and the slit 

 should be parallel, or that the images of slit and spectra shall 

 lie between the same horizontals in the field. One of the 

 gratings, Gr n , may now be moved parallel to itself by the 

 micrometer screw until the elliptic interferences appear. If 

 the plate M is not half silvered there are three groups of 

 these, as described in the preceding paper. Each group 

 passes from the initial degree of extreme fineness, through 

 maximum size, to a final degree, for a play of the screw of 

 about 1 mm. There is the usual radial motion of the fringes, 

 together with the drift through the spectrum as a whole. 

 To bring out the set of solitary ellipses, the silvered surface 

 of M should be towards the light and remote from the eye. 



