424 



Prof. Carl Barus on Interferometry 



The face of the grating is here supposed to be away from the 

 incident raj, as shown at gg in fig. 4. 



Kff. 4. 



Then it follows, as in paragraph 6, mid. mut., that the path 

 difference is (if y=y m -y n ) 



nk 



2y cos a + juue 



{ 



2 cos a 2 cos a cos (r — R) l + cos(r — /? 



COS?' 



-f 



cos r 



cos it 



= -2 i /cos« + £(Z 1 -Z 2 + Z 3 ), 



where n is the order of interference (whole number). This 

 equation is intrinsically simpler than equation (6), since 

 /ji r — yu R as already stated, and since a. is constant for all 

 colours, or all values of /jl and X in question. E. and r replace 

 B x and V 



In most respects the discussion of equation (11) is similar 

 to equation (6) and may be omitted in favour of the special 

 interpretation presently to be given. If fi=l, equation (11) 

 like equation (G) reduces to the case corresponding to e = 0, 

 with a different y normal to the grating. 



All the colours are superposed in the direct images of the 

 slit, H n and H (fig. 4), seen in the telescope, and the slit is 

 therefore white. This shows also that prismatic deviation 

 due to the plate of the grating (wedge) is inappreciable. 

 The colours appear, however, when the light of the slit is 

 analysed by the grating in the successive diffraction spectra, 

 D n or D, respectively. In equation (11) 3 /j, is a function 

 of X, and hence of the deviation 6 produced by the grating, 

 since 



sin (!-«)- sin = \/D. 



U} 



(11) 



