ACOUSTICS AND GRAVITATION. 71 



an object is virtually identified with the mirror M' and is displaced by its 

 micrometer-screw. The rays passing through the optical center of the 

 objective of the telescope thus become inclined to each other in proportion as 

 they are nearer, and the inclination angle vanishes only for objects at infinity. 

 Hence one component image will pass across the other slowly when the 

 micrometer moves, and this is attended with a change of fringes. 



Beautiful and intense fringes were obtained by the present method, dis- 

 played in the glare of an ocular out of focus and observed with a stationary tel- 

 escope. They remained equally serviceable when viewed with the revolving 

 telescope, where advantages in sharpness are often obtained by slightly reset- 

 ting the ocular. Displacements produced by moving the micrometer or the 

 revolving mirror were also examined with success. The effect was strengthened 

 by using a long-focus condenser in front cf the interferometei . 



If the device of obtaining fringes on images out of focus is rejected, the full 

 solar disk may also be used. This, however, requires large mirrors of strictly 

 optic plate in the interferometer and its accessories. Taking the solar diameter 

 at 0.53 and remembering that the distance 5 is doubled, mirrors 20 cm. square 

 would be needed for each 10 meters of 5, and the interferometer should be 

 placed close to the heliostat. 



65. The combined revolving mirror and telescope. This apparatus, which 

 has already been partially described in connection with figures 94 and 95, is 

 shown in figures 98 and 99, plan and elevation. Sunlight enters through the 

 long-focus doublet L, just in front of M, the focus being at the telescope, 10 or 

 more meters distant. The component beams leaving the mirror N f are 

 received by the mirrors n and n' in succession. The latter, being below the 

 disk ti' of the revolving mirror R, reflects them up vertically thi ough the ob- 

 jective P embedded in it! They are seen when P is in position, thiough the 

 eye-piece E. P' is a counterpoise, relative to the axis A of the electromotor. 

 The mirror n is at the intersection of a diameter of it' and the rays NN'. 



When the telescope is completed, the eye will see a composition of the 

 displacements due to the revolving mirror R and that of the objective P. 

 The latter moves tangentially to ti' and always in the same sense. The dis- 

 placement resulting from a turn of mirror may be taken as parallel to the face 

 of the mirror underneath it' and hence will be normal to the line nPP r 

 (diameter prolonged), or in other cases normal to the tangents ns' and ns" , 

 supposing the objective embedded at s' or s". 



It follows, therefore, that at P the displacements are opposite in direction, 

 and if they are equal there will be no displacement. With the objective at 

 P' the displacements are in the same sense, and the resultant equal to their 

 sum. At r's' and r"s", finally, the displacements are more or less at right angles 

 and the resultant a diagonal. 



Quantitatively, if /is the focal length of the telescope, a the distance of the 

 objective from the axis A, since the angle of reflection only is in question, 



