REVERSED AND NON-REVERSED SPECTRA. Ill 



61. Film grating. Another adjustment. The supernumerary spectra may 

 be gotten rid of altogether by using the method shown in figure 80. Here the 

 impinging vertical sheet of white light, L, from the collimator, falls upon the 

 clear or unruled part p of the plate of the grating, the film extending out as 

 far as shown at G. If M and N are the opaque mirrors, the reflected rays a and 

 b passing G are additionally reflected into a' and b', and thence, after leaving 

 the grating, into c and d. As both of the latter pass through the film, both 

 produce spectra; but b' and e may be blotted out by a screen at the mirror 

 N. This leaves only d beyond the grating. Again, the transmitted ray from 

 L, after reflection at M, is again reflected into c and d 1 ', which is made coinci- 

 dent with d. But c, being reflected from the unruled side, has no spectrum. 

 Thus the spectra due to the two rays d alone interfere. 



Had the grating been reversed, caet. par., then the ray c would have pro- 

 duced the strongest spectrum, and superposed on the other two it would 

 have greatly diminished the clearness. 



In the telescope, whereas the ray a' prolonged is white, the ray d' from M 

 and reflected from the film is strongly azure blue, due to regularly scattered 

 light. This blue image is apt to be less sharp, unless very flat parts of the 

 film are found. The two spectra, however, are good and the interferences 

 satisfactory. The sodium line is sufficiently indicated, though, like the blue 

 image, not quite sharp. 



This method of using the unruled edge of the plate of the grating for reflec- 

 tion is, of course, equally applicable and advantageous in the case of the 

 ruled grating. Only the two interfering spectra and no diffused light are 

 present in the field of the telescope, and if sunlight is used the Fraunhofer 

 lines are beautifully sharp. 



62. Equations. The equations for N e /e, for normal incidence I=R = o, 

 takes its simplest form as 



(i) Nc/e = M - Xd/z/dX =A+ 3-S/X 2 , nearly 



where N c is the coordinate of the center of a given ellipse on the micrometer 

 M, for the thickness of glass grating e, index of refraction ju, and color of wave- 

 length X. 



Hence if two different wave-lengths, X and X', are in question (5 refers to 

 differences), 



M 



8N e being the displacement of the micrometer to pass the center of ellipses 

 from line X to line X'. 



If n=A+B/\* and \d^/d\=-2B/\\ then 

 (3) 



