132 



THE INTERFEROMETRY OF 



This is curve a in figure 89. From it the mean rate 



- =0.47 cm./degree, or 27 cm./radian 

 Aa 



may be found. 



(2) Coarse large fringes. Smaller differential glass-path. 



0.1 0.2 C 



0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 



JVXio 3 =-25 +29 84 134 176 217 265 323 365 420 467 cm. 



This is the curve given (with double ordinates for distinction) in curve b, 

 figure 89, and in figure 90. Besides this the datum a= 0.6, N = 0.320 cm. 

 was obtained. In figure 90 the mean rate is 



AN 



- =0.465 cm./degree, or 26.6 cm./radian 



Aa 



agreeing with the preceding as closely as may be expected. We may thus 

 estimate AN = 2'jXio~* of displacement at the micrometer at H 3 per micro- 

 radian of turn a at the mirror m, which amounts to a little less than one 

 interference ring per micro-radian (about 0.2 second of arc) of turn. More- 

 over, turns of a less than a few degrees are certainly measurable. Through- 



8,4. 



-3' -4* -5 -6 



92 



iO W 20 cm 



out all this work the achromatic fringes are also available for precision in N, 

 but for this reason are more difficult to manipulate if individual fringes are 

 treated. They may, moreover, be much enlarged by rotating the mirror m 

 and advancing the micrometer at H$ in small steps in such a way as to pro- 

 duce contrary effects and thus keep the achromatic fringes in the field. If 

 the fringes leave the principal focus, the micrometer at Ha and its adjust- 

 ment screws may be actuated together in the same way. This is the most 

 available method for eliminating the glass-path, so that enormous spectrum 

 ellipses are obtainable. Finally, three groups of achromatic fringes (on 

 each side of the strong central group) were noticed, the distance apart of 

 groups corresponding to about AN = 0.0014 cm. 



