30 



THE INTERFEROMETRY OF 



11. Experiments continued. New interferometer. At the outset it was 

 necessary to ascertain the reason for the difference of the phenomena, as 

 obtained with one grating in paragraph 8 and with two gratings in para- 

 graph 10. As the probable cause is a lack of parallelism of the rulings in the 

 latter case, it was necessary to remount the second grating G' in the manner 

 shown in figure 19. Here A A is a baseboard, capable of sliding right or left 

 and of rotating on a horizontal axis parallel to the grating. The latter (in a 

 suitable frame) is held at the bottom by the axle, e, normal to the grating 

 and by the two set-screws a and b carried by the standards c and d. Thus 

 the grating could be rotated around an axis normal to its plane. At first a 

 Michelson plane-reflecting grating G' and a telescope were used, as in figure 

 16; but it was found preferable (fig. 20) to use a Rowland concave reflecting 

 grating G', with the strong lens at T, the grating receiving a beam of parallel 

 rays of light for each color from the collimator and first grating G. In this 

 case, with sufficiently high dispersion, a large, strong field was obtained, in 

 which even the very fine lines of the solar spectrum were quite sharp. Rotating 

 grating G' around a parallel horizontal axis, like AA, figure 19, made little 



a 



difference, relatively speaking; but rotation around the axis e, normal to its 

 plane, carried out by actuating a and b in opposite directions, made funda- 

 mental differences in the appearance of the phenomenon and eventually 

 suggested a new interferometer for homogeneous light. 



The adjustments are the same as in case of figure 1 6, G being the transparent 

 grating, except that G' is now a concave grating and T a strong eyepiece. 

 The distances G'T and GT were of the order of i and 2 meters. 



On rotating the grating G' on an axis normal to its face, from a position of 

 slight inclination of the rulings toward the left, through the vertical position, 

 to slight inclination to the right, the fringes passed through a great variety 

 o; forms, to be described in detail in 13 below. Difference of focal planes 

 between the Fraunhofer lines and the interferences were common, so that 

 effects of parallax were apt to occur. Thus when D* and D'z coincide, the 

 ladder-like phenomenon may lie between D'% and D'\\ or the ladder may pass 

 obliquely between the Dz D\ and D\ D'z doublets. The first experiment 

 with the new and powerful apparatus (plane transparent grating G, grating 

 space 35iXio~ 6 cm., and the concave reflecting grating G', grating space 

 173 Xicr 6 cm., fig. 20) was made with the object of verifying, if possible, the 



