148 DISPLACEMENT INTERFEROMETRY APPLIED TO 



Thus as o.i scale-part is given by the vernier, a film 0.03 mm. thick should still 

 insure an accuracy of one unit in the second place of M. Moreover, if Sx i, 

 the number of fringes is 



A fringe thus corresponds to 0.2 scale-part and is again capable of being 

 subdivided. 

 Tests would, therefore, be narrowed down to finding in what degree 



dx/x 8e/e 



is persistently zero, or dx/de constant, as e decreases indefinitely. It is again 

 improbable that the interferometer will register a sufficiently small e. 



An experiment made on the difference of an air-and-water film, figure 183, 

 devoid of spacing, showed Ax = 0.6 to 0.7 scale-part. On standardizing the 

 fringes, it was found that 5.8 fringes came to the scale-part. Hence the increase 

 8p. of refraction due to the film is equivalent to o.65X5-8 = 3- 8 fringes. If 

 for water 6^ = 0.33, equations (2) and (3) give us 



BA and CA, figure 183, not being optic plate, do not approach closer than this. 

 The strips were now left in place and a film of air and ether and water were 

 compared in succession. In this case it was possible to see the ether evaporate 

 while the fringes changed from the air to the ether position. The water does 

 not so easily evaporate between the plates of glass, hours being necessary be- 

 fore it is gone. The mean results of four experiments, each corresponding to 

 fixed plates, were 



Ether, AX=I.IO Water, Ax = o.99 



We may with the above equations (approximately) put 



0-333 



/*' 



So that for ether M = i .37 . This is quite as near the actual index of ether (M = 

 1.36) as the work with such thin films will admit. And yet the films are not 

 thin enough, probably, to suggest an increase of /*, as a result of the Twyman 

 surface effect. The interferometer, so far as I see, is thus not adapted for 

 work of this kind. 



IV. COMPARISON OF TWO INDEPENDENT SETS OF FRINGES. 



116. Apparatus. This experiment was also made with the self-adjusting 

 interferometer, for the purpose of obtaining two juxtaposed sets of fringes, of 

 nearly the same size, one pair being used as a sort of vernier on the other. If 

 converted into overlapping spectrum fringes, these should appear and vanish 

 periodically. Another method is given in 62. 



The apparatus is shown in figure 184, where MM, NN' are vertical mirrors 

 ' half -silvered), L, L' two sources of white light, C a Billet wedge compensa- 



