70 DISPLACEMENT INTERFEROMETRY APPLIED TO 



The following values were obtained : 



/ s I io 4 e s io 4 k d/f miles d//feet 



i i i.o 75 25 3.0 .016 82 



i 1.4 .75 62 25 2.5 17 90 



i 2.0 .5 37 25 1.4 15 77 



i .7 1.5 100 25 4.0 14 74 



The data are rough, as the tangent screw micrometer was not adapted for such- 

 small angles. Thus at 10 rotations per second the distance d traversed by light 

 per fringe is about 80 feet, or for the larger fringes 40 feet, per ocular scale- 

 part of o.i mm. It would be possible to read within this. Since the distance 

 5 is passed twice, d = 25; so that 5 = 20 feet per scale-part per 10 rotations per 

 second, may be taken as an experimental estimate. It is of the same order, 

 30 feet per fringe, as the theoretical value above. 



64. Experiments with the rotating telescope. Fringes on washed images. 



Having found the fringes for a short distance S, the revolving mirror was left 

 at rest and the revolving telescope, figure 96, tested for adequacy of light. 

 With the collimator and a long- focus condensing lens to illuminate the slit, the 

 fringes were seen distinctly and manipulated; but they appeared too faint 

 for practical purposes. 



Removing the collimator and lens completely and using sunlight directly, 

 the fringes (after adjustment) were displayed strongly on the intense images 

 of the heliostat mirror. In the revolving telescope they came out beautifully, 

 easily recording small micrometer displacements, or slight rotations of the 

 revolving mirror. 



At long distance 5 another difficulty arose* as the heliostat image (except 

 for mirrors exceptionally large) was apt to be point-like. The light, however, 

 was still abundant. Hence the device was adopted of putting the ocular out 

 of focus. A large white field or glare is thus obtained, quite satisfactory for 

 the display of fringes. 



A variety of important experiments was now made, relating to fringes 

 obtained on images out of focus ; for this glare may easily be enlarged to fill 

 the whole field by drawing out the ocular, or the reverse. It is necessary, of 

 course, that the two component washed images be in coincidence; and this 

 without a guide as to the identification of the points of their areas is a difficult 

 adjustment to make. But if precautions are taken that the two pencils 

 entering the objective of the telescope are quite coincident, the fringes will be 

 large and they will not vanish in any position of the ocular. The reason 

 for this is clear, since parallel pencils, partially coincident on entering the 

 telescope, will coincide at the focus of the objective only. Thus the fringes 

 will soon vanish for other positions of the ocular than the one corresponding 

 to this focus. 



One difficulty occurs here which must be guarded against. An ocular drawn 

 in or out from the principal focus implies an object at a finite distance. Such 



