836 The Indices of Doubly-refracting Crystals. 



value of b should have been in each case put 



m = M. and A i B (AN/« + l)=S; 



then (M-S)\ 2 =-2&. . ..... (11) 



The values of b so obtained show a very appreciable 

 variation in the march of b from the to the F line, viz., 



Line C D E b F 



Mean&xlO" 10 . -446 -421 -403 -405 *398 



If the mean value of b is computed from Ma scarfs data *, 

 between the C and D and the D and E lines, and compared 

 with the computed mean of the table, the results are 



C to D. D to E. 



Mascart bx 10- 10 = -421 -407 



Interferometer ... bx 10- 10 = '433 '412 



a difference of about 3 and 1 per cent., respectively. But 

 the C line in the grating interferometer is difficult to observe, 

 as the stationary interferences are very prominent in the red. 

 Between D and E, however, the agreement is as close as the 

 method warrants. 



6. Conclusion. — The above results, taken as a whole, bear 

 out the surmise that it must be possible to determine the 

 refraction of solid and liquid media by means of displace- 

 ment interferometry, with any degree of accuracy desirable, 

 depending ultimately on the length of the refracting column 

 employed. The centre of ellipses is never lost but may be 

 brought back to the fiducial Fraunhofer line, if the micro- 

 meter screw is sufficiently long and the end faces of the 

 medium are plane parallel. If the latter is not the case, and 

 the angle of wedge is not too large, the discrepancy may be 

 corrected by compensation. For this purpose the distant 

 mirror N (not the one at the micrometer M and controlling 

 the beam passing through the medium) is to be rotated to 

 restore the ellipses. 



The coefficient d/uu/dX may be obtained by the same method 

 with an adequate degree of accuracy, by using successive 

 Fraunhofer lines. In the case of work of precision, the 

 Cauchy equation as far as the fourth power of wave-length 

 is necessary, and three Fraunhofer lines are needed to deter- 

 mine the two essential constants. For this purpose it is 

 immaterial whether the plate is slightly wedge-shaped or 

 not. 



* See Landolt and Bomstein's Tables. 



