REVERSED AND NON-REVERSED SPECTRA. 



57 



refracting glass. In contrast with the ease in which the adjustments were 

 made with the symmetrical 45 prisms, the corresponding work with 60 prisms 

 proved to be exceedingly difficult. True, the prisms were large, including 

 long glass-paths. The sodium lines, now clearly separated, were markedly 

 curved, so that on placing them near together they either assumed an O-shape 

 or an X-shape. But the spectra were brilliant and nothing appeared to mili- 

 tate against a successful result. But it was not until after days of searching 

 that the fringes were found, and then only with two prisms of different, highly 

 refracting glass. They were not quite uniform, but it seemed impossible to 

 improve them. The ranges of displacement were found to be 0.088 to 0.093 

 cm. with the electric arc, 0.103 cm. with (condensed) sunlight. This is again 

 in accord with the large increase of dispersion, the range of displacement being 

 about five times greater than was the case with 45 prism of less refracting 

 glass. 



sf> 37 



25. Diffraction at M, N, replacing reflection. The present method of 

 observing interferences in the zero, first, second, third, and even fourth 

 order, successively, without essential change of the parts of the apparatus, is 

 noteworthy. I happened to possess a plane reflecting grating (D X lo" 6 = 200) , 

 cut into two equal parts by a section parallel to the rulings, and it was there- 

 fore easy to devise the method. In figure 

 38, the incident light L from the colli- 

 mator is separated into two component 

 beams a and a' by the 60 prism P. This 

 is essential here, as an abundance of light 

 is needed (sunlight should be focused by 

 a large lens of long focus (5 feet) on the 

 slit). The rays a, a' are then either 

 reflected or diffracted in any order by the 

 plane reflecting gratings G, G' into the collinear rays b, b'. They are then 

 reflected by the silvered right-angled prism P' and observed in a telescope 

 at T. G and G' and if possible also P' should be on micrometers, so that 



