REVERSED AND NON-REVERSED SPECTRA. 



55 



After these experiments an attempt was made to obtain similar results with 

 the more comprehensive method of two gratings (transmitting G and reflecting 

 G'} of figure 3, above. But here the choice of gratings with appropriate con- 

 stants was limited and with high double dispersion the fields were apt to be 

 too dark. Good results were obtained with the 60 prism and concave grating 

 and with the ruled grating together with the latter. In the method of figure 3, 

 the second angle of diffraction is necessarily greater than the first, 0'> 6. To 

 obviate this difficulty the method was modified for prisms as in figure 20, 

 where G is the concave grating, T a strong lens near its focus, and m and n 

 auxiliary mirrors. If this method is used for highly dispersive gratings (G 

 replacing P), the rays must be crossed as shown in figure 34. The fringes 

 were found in this case when G was a film grating, but the work had to be 

 abandoned, as the spectra were dull. 



The data given in table n again show marked increase of displacement 

 with the dispersion d6/d\, though it is not proportional here. The method 

 with two gratings lacks the brilliancy of the prism method. 



TABLE n. Range of displacement for different dispersive powers. Method of figure 20, 



full displacement. 



24. Normal displacement of mirrors (5 = o). This desideratum was secured 

 in the original methods, in which a single grating was used for both diffractions. 

 Rays in such a case have to cross the grating somewhat obliquely to the hori- 

 zontal. The method, furthermore, is restricted to the linear vertical fringes, 

 which are not useful if practical measurement is aimed at. 



In the methods with a right-angled reflecting prism (fig. 14), this result is 

 easily secured by displacing the prism. In all other methods (5>o), the dis- 

 placement of mirror is accompanied by sliding of the pencil along it. The 

 effect, as has been shown, is not negligible. It therefore seemed desirable to 

 devise other methods in which 6 = 0, and figure 35 is a device of this kind. 



Here G and G' are two identical gratings, the first, G, receiving the light L 

 from a collimator. The component pencils a, a' pass through the half -silvered 

 plate H, and thence (b, b') to the opaque mirrors M and N, one or both on a 

 micrometer. The pencils b and b', impinging normally, retrace their paths 

 and are thereafter reflected at the plate H into c and c' . These strike the 

 second grating G' at the proper angle of diffraction and thereafter enter the 

 telescope T together. The path of rays is symmetrical throughout. White 

 light is screened off at d. Reversed spectra are seen at T. Unfortunately the 

 only identical gratings at my disposal were film gratings of high dispersion 



