Lic/ht at certain Metal- Liquid Surfaces. 241 



however, that the rapid overflowing of the mercury would 

 tend to carry away any such film and leave a more perfect 

 contact between the two media. And from the final result 

 of these experiments it would seem that this expectation is 

 fully justified. During the course of a set of observations 

 under a liquid it was of course necessary to close the over- 

 flow tube (D) ; consequently the cylinder (C) was made 

 large enough to hold all of the mercury which it was neces- 

 sary to allow to overflow in the course of such a set. The 

 liquid thus displaced from the box (E) was made to overflow 

 at one corner and was caught in a suitable vessel. 



In order to avoid any deviation of the beam of light by 

 refraction at the free surface of the liquid, glass tubes 

 terminated by plane parallel glass ends were carried in 

 suitable brass holders by both the collimator and telescope 

 arms; when the mirror was in position, the ends of these 

 tubes were just submerged in the liquid in (E). The length 

 of the box (E) was made as great as the space between the 

 tube-holders would allow, in order to obtain as large an 

 angle of incidence as possible. The space thus available 

 was, however, too small to permit the use of angles of inci- 

 dence greater than 60°, which, though lessening the pre- 

 cision attainable in this kind of experiment, still yielded 

 ample accuracy for the ends in view. 



The apparatus for measuring the phase-difference and 

 azimuth of restored polarization was of a familiar type and 

 needs bnt brief description. The spectrometer * was one by 

 the ISociete Genevoise, arranged so that the table could be 

 swung to a vertieal position to accommodate a horizontal 

 mirror. The collimator arm carried the polarizing nicol 

 behind the lens ; the Babinet compensator was mounted 

 before the objective of the telescope; while the analysing 

 nicol was carried at the other end of the telescope tube. 

 The nicols and compensator came with the spectrometer and 

 were of good quality. By means of a second lens which 

 could be inserted behind the analyser, and an extra draw- 

 tube, the eyepiece could be focussed on the compensator 

 fringes. Since these are not definite objects on which to 

 focus, a small circular diaphragm w^as inserted behind the 

 last lens of the ocular in order to avoid a small residual 

 parallax which was otherwise necessarily present. These 

 modifications of the apparatus as originally received from 

 the Societe Genevoise and also the holders for the glass 

 tubes and the suface-cleaning apparatus described above, 



* This was obtained by the aid of a grant from the Bache fund of the 

 National Academy of Science. 



Phil. Mag. S. 6. Vol. 22. No. ]2S. Aug. 1911. R 



