308 G. Barus — Use of Compensators. 



ally. Very slight motion of the micrometer mirror M then 

 throws c either to the right or the left of the cross hair. In 

 this case the lens doublet at L is attached to the mirror and 

 moves with it, as stated. To obtain the extreme of sensitive- 

 ness, the path difference of NG and GM must be all but zero ; 

 i. e. the grating plate G and the lens doublet Z, fig. 4, must be 

 all but compensated for equal air distances, by the compensa- 

 tor C. In this case of full compensation the interferometer 

 pattern in the absence of a doublet L, would be enormous and 

 diffuse, seen preferably in the principal plane of the telescope, 

 but useless for adjustment. The introduction of a lenticular 

 compensator balanced by a compensator in GJY, transforms the 

 huge pattern into the small interference fringes in question, 

 with the advantage that the high mobility of the course design 

 has been retained. In other words, an index suitable for 

 measurements has been found, compatible with extreme sensi- 

 tiveness. In fact, it is difficult to place the micrometer mirror 

 M so that the region c, fig. 7, is exactly bisected. As the 

 plane in which these interferences are seen most distinctly is 

 1™ or more anterior to the principal focal plane, the Fraun- 

 hofer lines are unfortunately blurred and a cross hair is needed 

 as a line of reference. 



Brown University, Providence, R. I., July, 1915. 



