REVERSED AND NON-REVERSED SPECTRA. 117 



was obtained which passed from the initial horizontally long ellipse, through 

 circles, vertically long ellipses, vertical lines, into hyperbolic forms of increas- 

 ing eccentricity, as recorded in figure 82. 



On reversing the system, keeping the convex lens fixed near the mirror and 

 increasing the distance D by moving the other lens toward the grating, the 

 original ellipse usually flattened out further, as shown in figure 83. Moving 

 the lenses sideways parallel to themselves had no definite effect; moving 

 them fore and aft together (D constant) produced results similar to the above. 

 The vertical lines of figure 82 are liable to be sinuous or to resemble the grain 

 of wood around a knot. In case of figure 82, as the equivalent lens lies in 

 front of the mirror, the rays reaching the grating are thus necessarily converg- 

 ing. In figure 83 the equivalent lens lies behind the mirror, so that the rays 

 at the grating are more convergent. Both positions furnish essentially 

 convergent rays. 



If corresponding to figure 82, the convex lens is kept fixed near the grating 

 and the concave lens gradually moved up to it, the order of forms is reversed, 

 but not quite completely. They usually terminate in long, vertical ellipses, 

 before reaching which the wood-grained forms are sometimes passed. The 

 same is similarly true for the case of figure 83. 



With cylindrical lenses (respectively convex and concave, each i meter in 

 focal distance) very little effect was observed when the axes of the cylinders 

 were parallel to the slit. With the axes perpendicular to the slit, the effects of 

 spherical lenses were virtually reproduced, except that the central fields 

 partook of a more rectangular character. 



To carry out the purposes of the present paper with strong lenses, respec- 

 tively convex and concave, the vertical sheet of light from the slit must be 

 diverged into a wedge by the concave lens and then collimated by the convex 

 lens. The mirror, normal to the rays, reflects them, so that they retrace their 

 path and become a sheet of light before the final reflection and diffraction at 

 the grating. The following experiments were made with strong lenses: 



At first lenses of double the preceding focal power, /= 50 cm., were tried, 

 but with no essential difference in the results. Thereupon strong lenses 

 of focal distances /:= 73 cm. and / 2 = 13.1 cm. were used together, the 

 convex lens being, as usual, near the mirror. For .0 = 7.5 cm., about, these 

 gave fairly clear images of the slit and it was easy to find the ellipses, which 

 were now very eccentric, almost spindle-shaped in form. They could be 

 obtained strong and clear without difficulty, and the nearly horizontal lines 

 filled the whole spectrum. Reversal of lenses practically failed to give results, 

 the rays after reflection being too divergent. 



On the large interferometer, where the distances between mirror and grating 

 are nearly 2 meters, adjustment was more difficult and the result (if parallel 

 rays are retained) less satisfactory, because the slit images are not in focus at 

 the same time. This is particularly the case when the convex lens is nearest 

 the mirror and the concave lens toward the grating. Thus when/= 100 cm. 

 and D = 1 5 cm., the modified slit image may be twice as large as the other and 



