10 



THE INTERFEROMETRY OF 



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 i cm. But they were still too mobile to be used individually. 



The same design, figure 3, was now mounted on a round, heavy block of 

 cast iron B, 30 cm. in diameter and 4 cm. thick, the distance G to MN being 

 about 20 cm. A number of screw r -sockets, b, b, were drilled into B on the 

 right and left, for mounting subsidiary apparatus. G' as before was on the 

 universal slide (fig. 2), movable in the direction LT. The tablets t, t' , etc., 

 of G, M, N, and G' were mounted tentatively on standards of gas-pipe 1.5 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 horizontal 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 longi- 

 tudinal spectrum axes were both secured with advantage. Similar elastic 

 adjustments have been recently applied. It was now possible to manipulate 

 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-difference 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 reducing the rotation of the micrometer-screw the latter could 

 be used at once. 



In case of homogeneous light and a wide slit, fringes were visible in an ordi- 

 nary telescope for a play of over 2 cm. of the micrometer-screw, passing, how- 

 ever, between extremes of fineness. The slit images are not of equal breadth, 

 if first- and second-order spectra are superposed ; but if the longitudinal axes 

 are coincident, any position of the narrow image within the broader produces 

 a wide vertical distribution of fringes, usually more or less horizontal. They 

 are very easily found. The sodium flame is too feeble for use. The mercury 



