72 



DISPLACEMENT INTERFEROMETRY APPLIED TO 



the displacement will be/ojAJ due to rotation of mirror and waA* due to shift 

 of the objective. Hence, if f = a, the image at P will be stationary. At P f 

 it would move with the speed w(a+/). 



These conditions are easily tested by putting a fixed, vertical meshwork of 

 wire gauze (sieve) between the doublet D and the mirror M of figure 98 and 

 focussing the eye-piece E upon it. If a =/ the meshwork will remain stationary 

 and clear, however fast the revolving mirror rotates, for any directions of the 

 wires of the meshwork. In the same way a fine slit will remain in position dur- 

 ing rotation. Moving the wire gauze between M and D, the position of maxi- 

 mum definition is easily found. With higher magnification, finer diagonal 

 lines often appear, superimposed on the image. 



In case of the objective positions S'r' and S"r", the wires of the grating are 

 to be placed at 45 to the horizontal. In such a case one or the other of the 

 parallel sets comes out strongly on rotation, the resultant displacement being 

 along their direction. In these experiments one must distinguish between the 

 mere displacement of the spot of light which depends largely on the effective 

 area of the mirrors, and the displacement of an actual image like that of the 

 grating, the latter being here alone in question. 



The position S'"r'" is also interesting, inasmuch as here the speed of the 

 revolving mirror is virtually doubled; but no experiments were made with it. 

 Obviously the mirrors N and N' must be screened off to admit of the passage of 



the parallel pencils at the same time only. Otherwise there will be three reflec- 

 tions in succession, one from N, one from N and N', and one from N'- The 

 first and last are single pencils and are not desirable. 



Removing the objective from it' and using the revolving mirror in connection 

 with the apparatus, figure 96, not synchronized, the result was like a display 

 of occasional shooting stars, moving in parallel in all parts of the field, as 

 would be supposed; but the light conditions were not unsatisfactory. 



The combined plan just sketched is interesting for experimental purposes; 

 but it does not meet the conditions of the problem, as may be indicated as 

 follows: In the first place, it is desirable that the fringes be vertical and to 

 move with the rotating mirror, therefore, in the horizontal direction. This in 

 figure 98 implies fringes moving in the direction r. Hence the positions sr 

 and s'"r'" in the figure are not available; for the fringes would overlap in their 

 motion and vanish. In the position s'r' and s"r", however, the fringes moving 



