504 G Bar us —Application of the Displacement 



of the grating does not interfere with the definition of the 

 ellipses of the interference pattern. For any other distance 

 of M and N except these radii of curvature, the spectra in the 

 telescope will cease to coincide horizontally on rotating the 

 grating and the ellipses would at once vanish. On the 

 other hand, the displacement of the grating in arc at the 

 end of the arm of the horizontal pendulum, is registered in 

 amount by the shifting of the ellipses in the interfering 

 spectra. This displacement includes of course (as a small cor- 

 rection) the additional thickness of glass introduced by the 

 rotation of the grating. The displacement in question is the 

 arc, which, when referred to the axis of the horizontal pen- 

 dulum, measures its angular deviation resulting from the in- 

 clination of the earth's surface relatively to the plumb line. 



It is convenient to exhibit the details of the instrument 

 (figs. 2 and 3) in separate parts for convenience in drawing, these 

 being superimposed in practice. 



Fig. 2 shows the attachment of the two concave mirrors 

 J/^and iVof the interferometer to the pier P. Here abed is 

 an ordinary framework of 1/4 inch gas pipe. The end a is 

 firmty plastered into the pier, b rises at a slight angle, cd being 

 horizontal arid parallel to the pencil of light from the slit, 

 while g shows the position of the grating on the horizontal 

 pendulum in fig. 1. The arm b lies below the case in that 

 figure and is free from it. Each of the mirrors M and iV^is 

 . on plane dot slot adjustments, and M is provided with aFraun- 

 hofer micrometer suggested in the figure. Both M and JV 

 can be rotated around horizontal and vertical axes for adjust- 

 ment, the former, M, being provided with a fine motion. The 

 clutch e and the corresponding one for M (not shown) allow 

 the micrometer to be placed at a greater or less distance from 

 the grating. The center of the mirror is about on the same 

 horizontal level as the grating. It is also usually con- 

 venient to place the lens L of the collimator in its screen, on 

 the same rod cd, witli an appropriate clutch and rack and 

 pinion. 



The complementary frame work is shown in fig. 3 and holds 

 the slits of the collimator (or the filament of a Nernst lamp) and 

 the two telescopes T and T' in place for observation, T being 

 used for the direct slit image and T' for the diffraction spectra 

 and interferences. The frame fghi is as before of gas pipe, 

 f being firmly plastered into the wall on the front face of the 

 pier (the other one, ab, being on the side). The telescopes 

 J 7 and T' are necessarily adjustable on a horizontal and vertical 

 axis, may be raised and lowered and moved right and left 

 along the rod kl, held by a firm clutch at h. The lens L may 

 also be carried on ab, as has been stated. Right and left, up 



