REVERSED AND NON-REVERSED SPECTRA. 



21 



special advantage. Hence the fore-and-aft displacement e of the grating G' 

 will probably be accompanied by a slow motion of the fringes, from which 

 the angle a <p may be computed. 



The experiments recorded in table 7 were made with the grating G' on a 

 micrometer-slide moving normally to the face of the grating. With the 

 mirrors, etc., placed so that optical paths were nearly equal, the adjustment 

 screws on M and N sufficed to bring the fringes strongly into view. Succes- 

 sions of 3 and of 4 fringes were tested, as these required an adequately large 

 displacement of the micrometer, which was moved both forward and backward. 



The mean of the three results is 8e = 0.008 cm. per fringe. The data are 

 not smooth, because the micrometer placed between the mirrors M and N is 

 in an inconvenient position for manipulation. The different sets of values, 

 moreover, correspond to different adjustments and therefore to slightly differ- 

 ent values of a. <p . As an order of values only is wanted, it was not considered 

 worth while to remedy the deficiencies. 



In accordance with the equation given, if 5e = 0.008 cm., X = 58.gXio' 6 cm., 

 62= 20 be inserted, 



a 



X cos 2 6 2 



= - r- ~ =0.0095 radian = 0.54 

 2 de sin 6 2 



The adjustment is thus about half a degree out of symmetry, a result 

 which in case of improvised apparatus is inevitable and moreover without 

 significance in the precision of the method. 



12. Prism method. Reflection. The grating G was now removed and 

 replaced by a silvered prism, as shown in figure 9 (P, prism; M, N, mirrors; 

 G, grating; T, telescope). A small prism angle, <p, is essential (<p= 18, about), 

 as a large divergence of rays would 

 not be accommodated on the interfer- 

 ometer, figure 3 . The fringes were found 

 without difficulty, in the second order, 

 the arc lamp being used. They are also 

 easily distorted, if the edge of the prism 

 is not parallel to the rulings of the grat- 

 ing. In such a case the symmetrical 

 arrow-shaped forms become one-sided 

 and, as it were, curved or faintly 

 fringed beyond the limits of the strip . 

 To get the best adjustment, the lamp 

 should shed about the same amount of undeviated light from both faces of 

 the prism, on a screen temporarily placed behind it. The illuminated strips 

 on the grating must coincide to the eye while making the fore-and-aft adjust- 

 ment. Finally, the grating is to be slowly rotated on the axis normal to 

 itself, until fringes of satisfactory shape and size appear. Naturally this 

 is done through the telescope, and a readjustment of the longitudinal axes 



