146 



THE INTERFEROMETRY OF 



so that n and 5p are proportional quantities. The following results were 

 found : 



The observed data are the means of 5 or 6 trials. As it is difficult to observe 

 the rings without interruption in an agitated laboratory, there is no doubt 

 that observed and computed values are coincident. The first and last rings 

 are not easily counted, and individual data were found to agree with the 

 computed results perfectly. Finally, if the glass strain were effective (for 

 there is actual flexure), it would be shown in the observations made by pass- 

 ing the beam through different parts of the plate of glass, between the center 

 and the edge. No consistent difference was found. 



Hence an appreciable strain effect is also absent, and the reason for the 

 discrepancy in the two sets of values from AA 7 " and from n, in table 1 7 , remains 

 outstanding. 



89. Equations. In the preceding report * the equation for the value of 7 

 is deduced as 



log 



7 = 



or 



Here p and p are the pressures in the air-chamber (barometric) and the 

 vacuum chamber respectively, before exhaustion, # the original tempera- 

 ture of the air, AAf the displacement of the micrometer corresponding to the 

 shift of the ellipses on exhaustion. If the air-chamber is quite tight, AN" may 

 be taken at any time. C and i -\-x are the optic constants 



v* 1)= 952.6 



for dry air, being the optic gas constant, if /JL O i replaces p , the normal 

 density of the gas. To allow for the dispersion of air an empirical equation 

 (convenient in the present calculation), 



= 0.0002 ioi/X 341 



was constructed. The deduction assumes that the centers of ellipses are 

 brought back again to the fiducial line D, of the spectrum, the micrometer 

 displacement in question being AN. 



* Carnegie Inst. Wash. Pub. 149, Part n, pp. 166-168. 



