84 



Bar us — Displacement Interferometer. 



the front face of the grating (blue image due to scattering) is 

 next reflected at n, thence to m, whence it goes to JV and 

 returns by the same route, passing however from n through 

 the grating to the observer's telescope. The case surrounding 

 the electrometer must, therefore, be provided with a front and 

 rear window. With so many reflections (altogether ten, includ- 

 ing the one at the grating and the effect of the two windows) 

 this beam is considerably weakened, and it is advisable to use 

 black glass and not a silver mirror at the micrometer M. It is 

 preferable, moreover, to use the glass side of the mirrors 

 at m and n, as the silvering is liable to be brighter in the rear 

 and the effect of thin glass plate is of no consequence. This, 



Fig. 8. 



Fig. 9. 



o 



SI 



m 



^ II 



<r 



V 



Fig. 8. 

 Fig. 9. 



Quadrant electrometer with parallel mirrors. 



Diagram showing direction of rays, quadrant electrometer. 



in fact, was the greatest difficulty encountered, as the mirrors 

 m and n were not at first adequately silvered and polished. 

 Even when the direct image of the slit is clear the spectrum is 

 apt to be dull and the ellipses are hard to find. 



The adjustments, when so many conditions have to be met, 

 were not at first easy of attainment. The electrical installation 

 should first be completed and the needle in place between the 

 quadrants, the electrometer being placed so that the beam 

 from the interferometer strikes n (see figs. 8 and 9). A white 

 screen behind n, catching the light passing beyond the edges 

 and showing the shadow of n, facilitates this adjustment, the 



