15".] WITH REFLECTING GRATING. 129 



n:ay be rotated around a horizontal axis to place the lines parallel 

 to the slit of the collimator. The duplex grating is mounted on a 

 spectrometer as is usual for reflection. Finally each half may be 

 raised and lowered and moved horizontally to and fro, parallel to 

 itself, so that the half gratings when coplanar may approximately 

 reproduce the original grating. 



After each of the spectra are clear as to Fraunhofer lines, the 

 interferences here in question are produced by bringing these lines 

 (the D lines for instance) into perfect coincidence, horizontally and 

 vertically. Under these circumstances if the distance apart, r. is 

 suitably chosen, the interference fringes will appear throughout the 

 spectrum. These consist of an approximately equidistant series of 

 lines parallel to the slit, /. c, vertical lines, which are finer, ccct. par., 

 as the breadth of the crack at 5 between the gratings is larger. 

 They may be increased from the extreme fineness as they enter the 

 range of visibility to a maximum coarseness (in the above experi- 

 ments) of about three to five minutes per fringe, after which they 

 vanish. They cannot, in practice, be passed through infinite size ; 

 neither can they be produced symmetrically on the two sides of the 

 adjustment for infinite size. They cannot in other words be changed 

 from the positive to the negative condition of appearance. 



The occurrences are in fact as follows : if as in Fig 2, i > 6, 

 (parallel white rays coming from L and L', R and R' being reflected, 

 D and D' dififracted rays for the normal //), the grating g' must be 

 in advance or forward of ^. If now the airspace e is reduced micro- 

 metrically, g' retreating, the lines travel in a given direction ( from 

 left to right) through the spectrum, while at the same time they 

 grow continually larger until for a minimum value of c still positive, 

 they vanish as a whole. The period of indistinctness before evan- 

 escence is not marked. 



On the other hand if 0' > /' as in Fig. 3, the grating g' must be to 

 the rear of g and the air space c is throughout negative. If this is 

 now decreased numerically the lines travel through the spectrum 

 in the opposite direction to the preceding case, while at the same 

 time they coarsen until they vanish as a whole as before. The 

 grating g' is still behind g when this occurs. 



PROC. AMER. PHIL. SOC. L. 198 I, PRINTED .\PRIL 27, 191I. 



