68 



DISPLACEMENT INTERFEROMETRY APPLIED TO 



61. The revolving telescope objective. When R, in figures 94 and 9$, ro- 

 tates, mere flashes of light would be seen at T if stationary, and the fringes 

 would be invisible as they move broadsides on. Hence to study these phenom- 

 ena I first availed myself of what is virtually a rotating telescope, shown in 

 figure 96. H is the cylindrical body of a small electromotor, clamped at 

 the foot F, and B is the horizontal shaft of the armature. To this axle the 

 disk DD, about a foot in diameter, is attached in front and well balanced, 

 capable, therefore, of rapid rotation. DD carries the objective P of the 

 telescope, embedded near its outer edge, P' being a suitable counterpoise 

 at the same distance from the axis B. The eyepiece E and tube of the 

 telescope are carried by the ring GG, capable of rotating about the rear end 

 of the cylindrical bcdy of the motor and to be put in any desirable position by 

 a set-screw, as at c. E is thus stationary and the telescope is complete when 

 E and P are temporarily coaxial. 



The fringes, when found in the interferometer, figures 94 and 95, without 

 compensator, may be at any angle to the horizontal, and it is of ten troublesome 

 to change their inclination. Hence the eyepiece E is to be set so that the 



tangential motion of DD at E is parallel to the direction of the fringes. Since 

 the fringes move by virtue of the rotation of the mirror R normally to their 

 directions, they will be seen moving in the direction of the resultant. It is 

 necessary, therefoie, that the fringes be reduced to a length not exceeding 

 their breadth, practically to points. This may be done, for instance, by a slot 

 in a distant screen at the heliostat, the slot being at the proper angle to the 

 horizontal. These relations are shown in figure 97. Let a i, 2, 3 ... .a' 

 be successive positions of the achromatic fringe, corresponding to the positions 

 c, i, 2, 3 ... ,c' ', of the normal to the revolving mirror; let b, i, 2, 3 .... b f 

 be the corresponding positions of the same fringe resulting from the rotation 

 of the telescope objective. Then the fringe is seen in the line EE'. Hence 

 the retardation due to the passage of light twice over the distances S, S' will 

 produce a displacement of the line EE, observed in the telescope along the 

 direction dd', normal to it. It is in this direction, therefore, that an ocular 

 micrometer would have to be placed. 



