78 EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 



spectra of higher orders, with the two interfering spectra. For work of this 

 kind it would have been preferable to make / (angle of incidence) about 45. 

 The individual observations are given in table 9. Between D and E the error 

 is about 0.002 on a length of i2.6 = A7V (; for the total displacement; i.e., 

 about 1.5X10* of M i- In computing 8N C from equation (9), the value of 

 B computed for light crown glass from Kohlrausch's tables was accepted, 

 as a special measurement for so many plates of glass seemed out of the ques- 

 tion. The films of Canada balsam are negligible. 



The quantity hN c was roughly measured. To find this accurately it 

 would have been necessary to make special adjustments, as it is large (&N C 

 12. 6 cm., quite above the range of the micrometer), and as a readjustment of 

 the mirror must be made in the presence and absence of the column, for which 

 it is difficult to make an allowance. The end faces were not quite plane 

 parallel. Using equation (6) the value of the correction 2EB/\ Z is 0.605 cm., 

 whence 



at the D line. The value found directly from the total reflection for a similar 

 glass was 1.521. As without the correction /*= 1.551, the corroboration of 

 the equation is adequate. One may note that 2#/X 2 = 0.0265, as a correc- 

 tion of n, is independent of E, the length of the column. But, for purposes 

 like the present, a small thickness of glass (E about i cm. and within the range 

 of the micrometer screw) is preferable, even if the accuracy could be enhanced 

 by using a stronger telescope. 



Table 9 shows that the computed values of dN c happen to coincide with 

 the observed values between the D and E lines. Between D and C, D and b, 

 the results are quite within the errors of observation and satisfactory. The 

 F line was obviously not observed, some other line in this dark part of the 

 spectrum being mistaken for it. Thus the line X = 49. 5 8 would give 6^ = 0.108, 

 the line X = 50.41 would give 0.064, each coming close to some of the observa- 

 tions. The results as a whole therefore attest the accuracy of the equation 

 used, as the computed lines are clearly better than the observed lines. 



44. Observations. Blue glass column. Although this column was more 

 colored than the other, the observations were apparently not inferior. Table 

 10 contains the results. It was just possible to reach the F line, visually. As 

 before, these data reduce largely to the shift from D to E. The column was 

 too long to be compassed by the contact lever used, and the length E given is 

 therefore approximate. 



To compute n from &N C = 14.0 cm., observed, the equation is as before 



where the last term is 0.0265 f r the same B and X; hence 



^ = 1+0.5503-0.0265 = 1.5248 

 agreeing sufficiently with the experimental result. 



