146 



DISPLACEMENT INTERFEROMETRY APPLIED TO 



/= 1.5 4.5 9.4 1.5 am. 



io 6 AAf = 15 10 10 5 cm. 



This means, no doubt, that the residual torsion left by the last experiments 

 is being gradually eliminated by the longitudinal field. For the relation to / 

 is merely that of a decay. The field of the helix was H = 2-ji within, but it 

 covered less than one-half the tube. At io amperes the mean field was prob- 

 ably less than 100 gausses. Nevertheless, I can not assert, in the lapse of time 

 and many applications, to have quite eliminated the torsion strain. A per- 

 sistent displacement of one or two fringes (set) may have remained to the 

 end, with each reversal of current ; for violent commotion due to the longitudi- 

 nal strain here interferes with the measurement. 



III. LIQUID REFRACTION NEAR A SOLID SURFACE. 



1 14. Methods and first experiment. An interesting discovery has recently 

 been announced by Mr. F. Twyman (Nature, Nov. 20, 1919, p. 3 X 5- v l- I0 4). 

 indicating a marked increase of the index of refraction of certain liquids at the 

 surface of contact with a polished glass surface. As the method of experiment 

 is not given, I was induced to make a few trials with the same end in view, 

 with the aid of the self-adjusting interferometer which happened to be 

 available. The advantage here is this, that separate installments are 

 immediately possible and require no long searching for fringes and that the 

 latter may be had in any size. At the same time, there is sufficient ray separa- 

 tion possible for all purposes of the present kind. 







\ 



AM, 



182 



The interferometer is shown in figure 182, receiving light at L, reflected by 

 the four mirrors M, M r , N, N', and observed in a telescope at T. CC is a long 

 (15 cm.) metallic cell about 1.5X1.5 cm. 2 in square section, with plate-glass 

 faces at the ends. Through it pass the two component rays ab, cd, of the inter- 

 ferometer. The liquid is introduced through a small hole h in the top of the 

 cell, closed by a glass plate. 



The cell CC is on a horizontal micrometer m, so that it may be moved trans- 

 versely to the rays ab, cd. By this means either one or the other of these rays 

 is brought as near the metallic edge of the cell as desirable, until the fringes 

 disappear with the extinction of the ray, the other being alone in the field. 



I examined benzol and ether but in neither case was there the slightest 

 slipping of fringes with the motion of the micrometer-screw m, up to the point 

 at which they vanished. The experiments were varied in many ways which 

 need not be detailed here. I did not, however, expect any positive result from 



