EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 95 



on the two sides of the very thin air film. Probably the reflections from the 

 film to glass and return constitute the weak lines, the strong lines not being 

 reflected at the film, but directly diffracted and transmitted. The reticulation 

 does not interfere with the sharpness of the strong lines. 



The case 2 + 5 appears for the micrometer reading at the mirror M, 2.775 

 cm., a final advance of 0.75 cm. Since the group 2 passes through the glass 

 twice and the group 5 but once, the path difference is now 6. The path 

 difference 2 5 will not occur, as the ray 6 has no spectrum. 



The present superposition 2 + 5 gives the only ellipses obtainable with their 

 centers in the field. They are sharp in line, but usually somewhat weaker 

 than desirable in practice, unless the other spectra are blotted out as suggested 

 in 53. In such a case they become quite as available as the ellipses from a 

 plate-glass grating. 



56. Continued. The groups 3+4 and 3+5. Both of these (see figs. 58 

 and 59) are interesting in case of the given grating. 



The case 3+4 appears when the micrometer reading at M is 1.275 cm -> 

 seeing that whereas one component ray passes through the glass 3/2 times, 

 the other passes but 1/2 times, the rays starting in the middle. The path 

 difference is thus again 5, but the interferences are thoroughly different, natu- 

 rally, since the front glass plate is in question. They are very coarse, strong, 

 revolving lines, curved when horizontal. The center is not far distant, but 

 outside of the field. What is very striking is the rapidity of their revolution. 

 They pass, almost at once, from vertical to horizontal and back to vertical 

 again, and are thus a sensitive criterion for the position of the mirror M on 

 the micrometer. 



Finally, the case 3 + 5, since each of the component rays passes through the 

 glass once, reproduces the compensated position 8 = 0, with the micrometer, 

 M at 2.025. The fringes are the same strong reticulated set described above. 

 Both reflections of the component rays take place at the same face. 



57. Centers of ellipses. To bring the centers of the ellipses into the field 

 if the former are near at hand, the observing telescope may be raised or 

 lowered, provided of course the vertical extent of the entering pencil of light 

 is larger than the diameter of the objective. 



If the center is far removed, however, the grating and simultaneously the 

 mirror N must be correspondingly inclined, so as to bring the entering pencil 

 of light from N back again into coincidence with the pencil from M. Thus 

 the coarse fringes with M at 1.25 cm., when thus explored, are found to be 

 very eccentric ellipses with the long axis vertical, which accounts for the rapid 

 rotation mentioned above. In other words, when the spot of light on N is 

 near the top (grating and N being reciprocally inclined), the field intersects 

 the top end of the major axes and the curvatures may be seen. When the spot 

 on N is near its bottom, the lower ends of the major axes may be seen. Finally, 

 when the spot on N has an intermediate position, the lines no longer rotate, 



