120 Prof. C. Barus on Elliptic and other 



As a rule the adjustment is difficult, as an extra condition is 

 imposed in the parallelism of the slit and the rulings of the 

 gratings. The ellipses are liable to be coarse with their 

 axes oblique, clearer in some parts of the spectrum than in 

 others, unless means are provided for placing the rulings 

 accurately parallel. Even when well adjusted they are rather 

 polygonal than rounded in their contours. They are about 

 as strong with non-silvered glass M as with half-silvered 

 glass ; but in view of the multiple spectra, the adjustment is 

 much more difficult in the former case. 



It has been suggested that the white slit images must 

 appear eccentrically in the direction R. Hence, if a special 

 telescope is directed in this line, the final adjustment is 

 reached on coincidence of the proper slit images, provided 

 the rulings of the gratings and the slit are parallel. 



For 6' > i the second series of interference spectra occurring 

 at D eccentrically are broader, but only on perfect adjustment 

 do they occur simultaneously with the other set. In fact,, 

 since for the preceding case i = 0, or 



2sini = X/D, 

 and in the presen case 



sin ! — sin i = \/D y 

 therefore 



sin #' = 3 sin i — 3 sin 0. 



There is also an available set in the second order to the 

 left of E. In the gratings used above, D lies in front of Gr«, 

 being nearer the E than the L direction. 



2. Inversion of the Method.— The occurrence of the un- 

 deviated ray R suggests another method. For if the white 

 ray R is reversed, i. e. comes from an eccentric collimator,, 

 slit images will be seen in telescopes at L and E, whereas 

 overlapping spectra will appear in the direction D' eccentri- 

 cally and in the lines R and R'. One of the latter may be 

 lost in the collimator. The former occurs for the same 

 angle 0', so that 



sin #' = 3 sin 2. 



Moreover, if 1 = 45° is the angle of incidence of L upon M 

 when sodium light is taken, so that 



0'=26° 14', i=$° 28', 



the R, D, D' rays make angles 2i, 6' -\-i, 6' — i, respectively , 

 with the E direction ; or the sum of the angles at D and D / 



