56 Messrs. C. and M. Barus on Interference of 



These data merely test the equations, as no special pains 

 were taken for accurate measurement, which neither the 

 micrometer screw nor the special adjustments warranted. 

 Usually the micrometer equivalent of 50 fringes was 

 observed on the screw. The maximum distance e between 

 grating and mirror was '48 cm. of glass and '25 cm. of air 

 conjointly, or within 1 cm. In the case of tine fringes mere 

 pressure on table or screw impaired the adjustment. 



8—9. Secondary Interferences. 



We come now to the consideration of the minor inter- 

 ferences (Table I.), which are either weaker, finer, or more 

 diffuse than those discussed. In the interpretation of these 

 we have not met with adequate success (assuming that after 

 two reflexions the fringes can no longer be seen) to give it 

 space here. We will therefore dismiss it with the remark 

 that each of the three incident rays of figure 1 corresponds 

 to three emergent ,rays for #>r and three for #<r. If we 

 call these a, b, c, a', b', c' f a", b'\ c" for either case, the possible 

 partial interferences may be found by grouping the terms of 

 the following determinant in pairs : — 



a b c 



a' b' c 

 a" b" o" 



There are 18 cases, most of which, however, are identical 

 in path-difference. 



10. Convergent and Divergent Rays. 



What finally characterizes the above groups of inter- 

 ferences is the difference in position of their focal planes. 

 They rarely coincide with the spectrum (parallel rays), and 

 hence do not always destroy it. If present with the spectrum 

 the latter is wholly wiped out. If the strong fringes are 

 convergent for a given adjustment of grating they become 

 divergent when the grating is rotated 180° about its normal. 

 Hence the plates of glass are sharply wedge-shaped, and to 

 these differences the location of the focal planes is to be 

 referred. 



In addition to this the three regular reflexions are not in 

 the same focus which shows the surfaces (collodion film) to 

 be slightly curved. The above experiments succeed best 

 when two of the reflexions are yeliow r ish, which probably 

 means that the grating face is from the observer. 



