I9II.] WITH REFLECTING GRATING. 127 



For d' > i the second series of interference spectra occurring at 

 D, eccentrically, are broader, but only on perfect adjustment do the\- 

 occur simultaneously with the other set. In fact, since for the pre- 

 ceding case i^=6, or 



2 sin i = X/D 

 and in the present case, 



sin 6' — sin / = X/D, 

 therefore 



sin 6' ^2> s"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 G,,. being nearer the E 

 than the L direction. 



2. Inz'cysion of the Method. — The occurrence of the undeviated 

 ray R suggests another method. For if the white ray R is reversed, 

 i. e., comes from an eccentric collimator, slit images w'ill be seen in 

 telescopes at L and £, whereas overlapping spectra will appear in 

 the direction D' eccentrically and in the lines R and R' . One of the 

 latter may be lost in the collimator. The former occurs for the 

 same angle 6' so that 



sin ^'^3 sin/. 



Moreover, if 7 = 45° is the angle of incidence of L upon M when 

 sodium light is taken, so that (9' = 26° 14'. / = 8° 28', the R, D. D' 

 rays make angles 2i, 6' -\- i, 0' — /, respectively, with the E direction ; 

 or the sum of the angles at D and D' with the E line is 2O' , their 

 difiference 21, and the rays D, R, D' intersect at a common centre on 

 Gm- Hence if we place the plane of Gm at the centre of the spherom- 

 eter and arrange -1/ and G„ eccentrically, the angles may be meas- 

 ured as before. 



3. Resolution of the Slit Image. — If the sharp white images of 

 the slit in a Michelson apparatus for the case in which the incident 

 light consists of parallel \vhite rays from a collimator, be accurately 

 superimposed and the opaque mirrors be set at the proper distances 

 from the semi-transparent mirror by the micrometer, the slit image 

 may itself be view^ed through a grating and will then show' elliptic 

 interferences in all the spectra. The apparatus is here eccentric, 



