Reflected- Diffracted and Diffracted- Reflected Rays. 55 



of equation (20' 7 ), producing a kind o£ alternation or flickering 

 which it is very difficult to analyse or interpret until e is 

 very small, when all three sets are broad and easily recognized. 

 Sunlight should be used. Nothing like these alternating 

 fringes is seen in the first order. 



The above equation shows finally that Be is not exactly 

 proportional to wave-length, though the former decreases 

 with the latter as found above. 



The three equations (20) indicate finally that for i^>0' 

 all fringes travel in the same direction with increasing e ; 

 whereas if 6' !>z, the set corresponding to equation (20) travel 

 in a direction opposite to that of the set (20') and (20"). This 

 is strikingly borne out by making the experiment for 6' !>z 

 with a small angle i, both in the first and second order, 

 flickering in the latter case is accentuated. 



Table II. contains a few data obtained by carrying the 

 mirror on a Fraunhofer micrometer, reading to '0001 cm., 

 toward a stationary grating film. Observations were made 

 in the region of the D lines. The grating was originally 

 between plates of glass g = *48 cm. thick. Finally the plate 

 between grating and mirror was removed, the whole distance 

 now being an air space. This has no effect on he, but e may 

 then be reduced to zero and the fringes enlarged. 



Table II. — Interferometer Measurements. Replica grating 

 (collodion film). Air space 0-*25 cm. ; total space 



0- 



'9 cm. 













i. 



e 





Coarse fringes. 



Fine fringes. 



^xio 6 



obs. 



£eXl0 G 

 calc. 



^xio 6 



obs. 



cexW 

 calc. 



o 

 22 



30 



o 

 2 



6 



cm. 

 390 



cm. 

 391 



cm. cm. 



45 







21 



1 



131 



130 





67 



30 



35 



1 



72 



68 





25 







4 



7 



322 



324 



— — 



30 



2 



8 



37 



241 



240 



— \ — 





Glass 



reino 



ved between mirror { 



md grating. 



22 



25 



1 



40 



387 



392 



1 34 1 {% 



45 







20 



49 



m 



130 



oo : |42 

 3o 1 32 



