14 



THE INTERFEROMETRY OF 



that it ran more smoothly. The following values were found in two repeti- 

 tions, from 4 pairs of 80 fringes each : 



o 6 = 3o.i, 30.0 cm. 



and in 5 pairs of 100 fringes each, 



o = 2Q.5 cm. 



If the mean value of these data is compounded with the above mean, the- 

 average is 



5^Xio 6 = 29-56 cm. 



5. Continued. Second=order spectra. The same phenomenon was not 

 sought in the two second-order spectra from G'. Magnificent arrows were 

 obtained, useful throughout about 5 mm. of the micrometer-screw, after which 

 they lost clearness. This limited range could no doubt be immensely increased 

 if optical plate glass were employed in place of the ordinary plate used. The 

 data for pairs of observations, including 60 or 80 fringes, were (5 repetitions) 

 5# X icr 6 = 30. 3, 30.5, 30.0, 30.8, 31.1 cm. per fringe, giving a mean value of 

 5tfXio 6 = 3o.5 cm. In the last two measurements the sodium doublets 

 coincided. 



In this case sin 0" 2 = 2\/D 2 , where D 2 = 200X10^ cm. and D\ = 352 X icr 6 cm. 

 (first grating). Thus 



(7 = 45 44', 5 = 2 628' 



If the above mean data are summarized the results appear as follows (X = 

 58.93 Xicr 6 cm.): 



G first order, G' first order, mean 8eXio 6 = 2g.$6 cm. 



G first order, G' first and second order, mean 5eXio 6 = 3o.2 cm. 



G first order, G' second order, mean 5eXio 6 = 3o.s cm. 



And if computed as 8e = \/2 cos 5/2, these become 



TABLE 3. 



The maximum error of 4X icr 7 cm. is equivalent to but a little over i per cent 

 of the distance between fringes, and it would be idle to suppose that the appa- 

 ratus, figure 4, could be set more accurately. In fact, the largest error occurs 

 in the second set, which were first made and in which the play of the apparatus 

 was inadequately smooth. 



6. Theory. Hence the theory of the apparatus (fig. 6) may be regarded 

 as justified. Here the rays Y and V come from the first grating (G transmit- 

 ting), and after reflection from the opaque mirrors M and N (the former on 

 a micrometer) impinge on the second reflecting grating G', with a smaller 



