0. Barus — Displacement Interferometer. 113 



Since the telescope _Z? rotates both around its own axis e" and 

 around the standard Q, elaborate centering of the grating G 

 is not usually necessary. The latter is mounted between strips 

 of cardboard or wood and secured by the screw g, the brass 

 clutches being about twice as far apart as the thickness of the 

 grating. In other words, the grating may be slightly moved in 

 a direction normal to itself. 



To adjust the parts, sunlight (preferably) or arc light is 

 passed into the widened slit of the collimator, in a dark room, 

 so that the spots falling on the mirrors M and J¥ (the grating 

 being suitably turned) and on the objective of the telescope E 

 are seen and the different reflected images brought nearly into 

 coincidence. A further adjustment is then made through the 

 telescope E, two of the usual four images of the slit (now nar- 

 rowed) being placed in coincidence horizontally and vertically 

 by manipulating the leveling screws on B. Specks of dust, or 

 nicks in the slit, greatly facilitate this adjustment. The tele- 

 scope is then turned to the diffraction spectrum, preferably of 

 the first order, and the drum actuated till the interferences 

 appear. Naturally the distances GN and GM are to be 

 approximately equal to begin with. The solitary ellipses are 

 best for general purposes and they usually correspond to unde- 

 viated yellowish and bluish single slit images. The multiple 

 slit image is to be avoided. If the rings are not quite centered 

 in the spectrum, they may be made so by cautiously adjusting 

 the screws at B, which tip the mirror about a horizontal axis. 

 The telescope may be moved with its foot sliding on a plane. 

 The three possible positions of the mirror JV (positive uncom- 

 pensated, self -compensated, negatively uncompensated) are 

 about l cm apart on the micrometer, for a plate of glass -68 cm 

 thick. When the arc lamp is used, the accentuated sodium 

 lines in the spectrum may be used in place of the white unde- 

 viated images of the slit, both for adjustment of the two 

 spectra for coincidence and as a fiducial mark, in place of the 

 cross hairs in the telescope. For a small angle of incidence the 

 sodium lines of higher orders of spectra are also liable to be 

 available. 



To measure the angle of incidence 7, the table Zis turned 

 in its socket, until the reflected image of the slit coincides with 

 the slit itself. A hole is cut in the top or side of the collima- 

 tor tube near the slit (not shown), for this purpose. There- 

 upon the table / is turned back again until the images coincide 

 in the telescope. The angle read off on the graduated plate K 

 is 7, the reflected ray travelling over 21. The table 1 is pro- 

 vided with an index and vernier (also omitted in the diagram). 



The apparatus described being made virtually of hollow 

 parts is light enough to be carried about with convenience. 



