EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 79 



The observed and computed values of 6N C show the same agreement within 

 the errors of observation as before. Differences are due to the value of B 

 used, which is naturally not quite the same as in the preceding case and should 

 here have been about 2 per cent, smaller. Between b and F, 

 would give 6.^ = 0.0709, which was probably observed. 



TABLE 10. Blue column. = 25.4 cm.; e = o.68cm.; 5=4.6X10-"; 7 = 15; -=9.7: 



45 Observations. Shorter column. The results for this column are given 

 in table 1 1 . It was less than one-third as long as the other columns, but, 

 absorbing less light, all the lines were seen. The ellipses being more mobile, 

 sharper adjustment is implied; but the F line could not be recognized with 

 certainty and there was difficulty at the C line. 



In this case, AA/" c = 3.g6i cm. lay within the compass of the micrometer. 

 The only error therefore is the intermediate readjustment of mirror in presence 

 and in absence of the column. The index of refraction of the D line is thus 



M = 



3.061 0.0265 

 ^675 ^ = '-5' 6 ' 



which is of the same order as before. 



TABLE n. Short column. = 7.1675 cm.; e = o.68 cm.; 5=4.6Xio- u ; 7 = 15; ^=9.7; 



The results for this series are scarcely as good as the preceding, relatively, 

 since finer micrometric measurement was required ; but, absolutely considered, 



