CHAPTER IV. 



REFRACTIVITY DETERMINED IRRESPECTIVE OF FORM BY DISPLACEMENT 



INTERFEROMETRY. 



48. Introductory. Some time ago* I made a number of experiments on 

 the use of curvilinear compensators in connection with the displacement 

 interferometer. It is obvious that the curvature in such a case must be very 

 small, so that single lenses for the purpose are not easily obtained. The use 

 of a doublet of two lenses of the same glass, but respectively convex and con- 

 cave, meets the case fairly well, the necessary refracting power being received 

 by spacing the doublet. Lenses of about i diopter each gave the best re- 

 sults, bringing out fringes of quasi-elliptic and hyperbolic symmetry in great 

 variety. 



Later it appeared as if plates of different varieties of glass, as for instance 

 crown and flint, if placed in the two component beams MH , NH, figure 54, 

 would produce the same phenomena. The flint plate used, however, proved 

 to be inadequately plane, so that the result is in doubt. 



More recently I have endeavored to secure similar results by submerging 

 the lens (convex or concave) in a liquid of about the same index of refraction. 

 This method would seem to be interesting in other respects, for it is probable 

 that the index of the solid may be determined in this way irrespective of 

 form.t If, for instance, the liquid and the solid have the same index, one 

 would be tempted to infer that the latter may be removed or inserted with- 

 out displacing the center of ellipses at the particular wave-length under con- 

 sideration. The index of the liquid in place is then determinable by the 

 interferometer to a few units in the fourth place. 



If experiments of the present kind are to be accurate, it is obvious that 

 the walls and cavity of the trough in which the lenses are to be submerged 

 must be optically plane parallel; otherwise some compensating adjustment 

 must be made at the opaque mirrors of the interferometer, and for this no 

 adequate allowance is at hand. It did not, however, seem worth while to 

 provide expensive apparatus before the method had been worked out in de- 

 tail. Accordingly the present experiments were conducted with troughs of 

 ordinary plate-glass put together by myself, and little attention will be given 

 to absolute values of index of refraction, as such. 



49. Preliminary experiments. The first experiments were made on a large 

 linear interferometer (see fig. 54) with distances of nearly 2 meters between 



* Carnegie Inst. Wash. Pub. No. 249, 1916, chapter ix; cf. Amer. Journ. Science, XL, 

 pp. 299-308, 1915. 



t Mr. R. W. Cheshire (Phil. Mag., xxxn, 1916, pp. 409-420) has recently used Topler's 

 method for the same purpose with marked success. 



95 



