ACOUSTICS AND GRAVITATION. 147 



this experiment, seeing that the range within which any such capillary effect 

 would be active would probably be too narrow to admit of visible fringes, 

 even for a thin blade of light at L. 



1 15. Second experiment. The next experiment was more promising. The 

 cell CC in figure 182 was removed and replaced by a plate of glass A A, figure 

 1 83 , normal to the rays ab, cd. This is without effect on the fringes. Two strips 

 of glass B and C of identical thickness were then mounted on opposite sides of 

 AA and held in place by wooden clips. Through them the rays ab and cd, 

 respectively, passed. The strips BC are without effect on the fringes if AA is 

 firmly clamped in a vertical plane. 



If, however, a drop of water or other liquid is placed at the plane of contact 

 of one strip only (at CA, for instance), the fringes are immediately displaced in 

 view of the entrance of a capillary film of liquid between A and C. Even with- 

 out any spacing inserted between C and A, the displacement is quite large, 

 four fringes for instance. 



These experiments are necessarily made with the achromatic fringes, since 

 the displacement is practically instantaneous. They may be obtained in any 

 size by rotating the mirrors N, N r , or both, around a horizontal axis. A vertical 

 axis is necessary to secure coincidence of slit-images. The rays ab, cd may be 

 separated at pleasure by sliding the mirrors M, M' (together) in the direction of 

 the rays. The fringes are necessarily horizontal and the index obtained holds 

 for mean wave-length. True, with the use of the spectro- telescope the corre- 

 sponding spectrum fringes are immediately at hand ; but it would not be easy 

 to determine their displacement, even with the clue given by the achromatics. 



For guidance as to the quantitative relations, the equations of the last report* 

 may be used. If one of the films is air, the index M= i zB/\- of the liquid, at 

 the wave-length X, for a thickness of liquid film e, is 



(1) ft = i - 2/X 2 -fC &x/e* 



where C is the constant of micrometer when its displacement reading is A#; 

 so that for n fringes 



(2) n\ = C&x 



Ax may be in arbitrary units, like those of the Billet wedge micrometer, for 

 instance. The usual value of the factor C is of the order of C = 3 X io~ 4 . Hence 

 in equation (i), M i-f 2J3/X 2 being constant for a given liquid and X, A# is 

 proportional to e. If the parenthesis is taken as 0.6, for instance, A* is 500 

 times larger than e. Again, on differentiating (i) apart from details 



and if 5x is one scale-part, edn = 3 X io~ 4 ; or 



e= 0.3 0.03 0.003 cm. 



'Carnegie Inst. Wash. Pub. No. 249, Part IV, p. 81. 1919. 



