Non-reversed Spectrum Interferometry . 



403 



being the micrometer mirror. The mean distance of M from N 

 was about 15 cm , from MN to G' about 10 cm and to G 40 cm . 

 Later these distances were enlarged. First order spectra were 

 used and the fringes obtained easily and brilliantly, particularly 

 with mercury light, in both green and yellow. They rotated 

 as above, admitted a displacement M of about l cm . But they 

 were still too mobile to be used individually. 



The same design, fig. 3, was now mounted on a round heavy 

 block of cast iron, B, 30 cm in diameter, and 4 cm thick, the dis- 

 tance G to MN being about 20 cm . A number of screw sockets 

 b, b, — , were drilled into B on the right and left, for mounting 

 subsidiary apparatus. G' as before was on the universal slide 



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(fig. 2), movable in the direction L T. The tablets, t, t', etc., 

 of G, M, N, and G' were mounted tentatively on standards of 

 gas pipe l # 6 cm in external diameter and 6 cm long. Slight 

 pressure by the finger tips showed a passage of several fringes 

 across the field, but the fringes were stationary in the absence 

 of manual interferences and in spite of all laboratory tremors. 

 A parallel arm of the same pipe was therefore firmly attached 

 to the stem of N and M, each arm terminating in a fine hori- 

 zontal set screw, s, s, below, adapted to push against the rim of 

 the iron block. In this way adequately stationary conditions 

 and an elastic fine adjustment for superposed longitudinal 

 spectrum axes were both secured with advantage. It was now 

 possible to manipxilate the micrometer at M by hand ; but a 

 glass plate compensator, C, rotated by a tangent screw over a 

 graduated arc was also convenient. Later other types were 

 attached, including an air compensator, in which path differ- 

 ence was secured by exhausting the air within a closed pipe 

 provided with glass plate ends. These contrivances were 

 eventually superfluous, however, as it was found that on reduc- 



