obtained by Combinations of Different Interferometers. 131 



lias an advantage over that with a fixed etalon. The order 

 of accuracy with which we can find a small difference in 

 wave-lengths is such that it is sometimes necessary to bring- 

 in the correction to the usual formula of reduction for the 

 difference in the order of spectrum from that at grazing 

 exit, although the observation is made very near it. 



There is, however, one doubtful point which requires 

 special mention. For observing the interference points 

 distinctly, the objective of the telescope or photographic 

 camera should not be focussed to infinity. This problem 

 has been discussed by Kolacek *, with results which make 

 the application of the usual formula of reduction somewhat 

 doubtful. 



Air-plate and Echelon Grating combined. — The crossed 

 spectra formed by this combination present special interest on 

 uccount of the ease with w T hich two spectrum lines can be 

 brought into coincidence. The photograph of the green 

 line with some of its satellites is shown in PI. I. fig. 3 ; 

 the principal line appears on both sides with nearly equal 

 intensity. The arrangement of interference points is not so 

 simple as with the combination before mentioned, but it has 

 the advantage of facilitating the measurement of the dif- 

 ference in wave-lengths, so that it cannot be overlooked in 

 spectrometry. 



Denoting the distance between the plates in the interfero- 

 meter by e, the order of interference for light of wave- 

 length X by P, which is an integer, generally very large, 

 and calling the angle subtended by the ring 20, which is 

 generally small, we have the relation 



2e 



=-COS0 = P. 

 A, 



•■Counting the rings from the centre, we have for the Ath ring 



A 



If we determine the diameters of m different rings, we have 



a P m(m-l) 2e% 



Z r h = m-r - — i. — - 2 cos On. 



Neglecting # 4 , and putting 



m 



2' 

 1 



* Kolacek, Ann. der Phys. xxxix. p. 1431 (1912). 

 K2 



