REVERSED AND NON-REVERSED SPECTRA. 73 



proved very trying, however, because the ruled faces of the available grat- 

 ings at M and N were but 0.5 inch square. There is thus, without refined 

 and special instrumental equipment, considerable difficulty in adjusting the 

 rays to this small surface. This was particularly true in the higher orders of 

 spectra. 



To obtain sufficient light the resolving grating G was replaced by a 60 

 prism. The dispersive powers are thus the same as in 25, Chapter I. The 

 work proceeded smoothly in the orders of o (reflection from grating face) 

 and i. In the third, the fringes were hard to find and hard to retain, for rea- 

 sons which I do not understand. There was abundance of light, except in 

 the fourth order, which was abandoned for that reason. The best results 



were * 



Order o, y = 0.066 cm. dd/d\= 760 



1, -230 3,500 



2, .450 6,400 



3, .650 9,900 



Though much time was spent on this work, the results (excepting the 

 first) are doubtless still too low. Since the path-difference zy corresponds 

 to x = 2e cos 5/2, cceteris paribus, and since the data in x, table 12, are essen- 

 tially half the total range (rotation but 90, while it is 180 here), y corre- 

 sponds to x. Thus the results in table 12 are larger throughout, but the 

 present data make a smoother series even through the third order. 



36. Spectra both reversed and inverted. This is an interesting combina- 

 tion of the two methods of investigation and not very difficult to produce. 

 Retaining the adjustment for inverted spectra as in 30, figure 43, the light 

 impinging on the grating G is dispersed, preferably by a direct-vision grating 

 (with auxiliary prism). The rulings of both gratings (the prism grating g 

 inserted as shown being between the collimator at some distance and the 

 grating of the interferometer G, figure 43) must be parallel. If the grating 

 constants are different (D = 167X10^ cm. film and .0 = 352X1 cr 6 ruled 

 grating were employed), the spectra in the telescope are naturally of different 

 lengths ; for the dispersion of the prism grating g is increased on one side and 

 decreased on the other side by the second grating G. Moreover, this decrease 

 from the larger dispersion of the first grating g is beyond zero (achromatism) 

 into negative values. Hence, the corresponding duplicate spectrum in the 

 telescope is a small and a large spectrum reversed, while the inversion re- 

 mains intact. In the experiment made, the larger D\D'z distance was some- 

 what more than twice the smaller D\D Z . 



It is now merely necessary to place any longitudinal axis (line of symmetry) 

 of the spectra in contact, or it is but necessary that the spectra are longitud- 

 inally parallel and overlap. The phenomenon a, figure 50, then appears 

 at the intersection of the lines of longitudinal and of transverse symmetry. 

 It is thus proportionately nearer the smaller DiD 2 and farther from the 

 larger D'iD' 2 doublets, but always between them. If the DiD 2 lie within 

 the D'iD'z lines, the fringes lie within the DiD 2 pair. 



