DISPLACEMENT INTERFEROMETRY. 75 



If the linear phenomenon of reversed spectra, coinciding on a certain line 

 of the spectrum, is wanted, a prism may be inserted between m and m f suit- 

 ably adjusted. These fringes then also appear at once and may be put in 

 any color, at pleasure, on rotating any mirror, say N, on a vertical axis. 

 Rotation around a horizontal axis enlarges them. 



Finally, if separate plate-glass compensators are placed in the paired beams 

 8 5 and 2 I, for instance, and rotated around a horizontal axis, independently, 

 the fringes may be displaced up or down the slit-image for the purpose of 

 measurement. A double-offset air-compensator, consisting of 3 right-angled 

 V-mirrors with their corresponding faces parallel, the central V-mirror movable 

 on a micrometer (described in my paper on gravitation) is available. Such a 

 compensator would be placed normally to the rays 8 5 and 21, for instance, 

 to give them lateral path-length. In these cases the spectre-telescope may 

 also be used where the strong bands register the displacement in any wave- 

 length. Since the slit may be broad there is a great abundance of light. 



The rays 8 5 and 2 I may be made of almost any reasonable length and 

 distance apart, if the mirrors N, N', m, m' are broad. To secure greater length 

 the mirrors m, m' (rigidly connected) may be moved at pleasure in the direc- 

 tion 5 8, without disturbing the fringes, good slides presupposed. The rays 

 may be separated, if x is the available breadth of mirror, to an extent x cos 45, 

 by moving the rigid system NN' in the direction 8 5. 



If either of the mirrors m or m' is separated (as, for instance, at a in fig. 79), 

 the part may be placed on a micrometer; but the apparatus would not then 

 be quite self-adjusting, for the parts will not in general be coplanar. But 

 facility in adjustment is nevertheless increased. 



To secure the best conditions for sharp, strong fringes the two slit-images 

 seen in the telescope must be of equal intensity, and this depends on the half- 

 silver N. On testing a number of plates it is usually easy to find one which 

 fulfills this condition nearly enough. The fringes are still good, even when 

 the intensity of images is noticeably different. The secondary images due to 

 the reflection from glass faces are either weak or (if thick plates are used) 

 these reflections may be blotted out by small opaque screens suitably fixed 

 to the mirrors m and N. It is interesting to observe that with proper adjust- 

 ment these secondary reflections carry their own fringes, some of which are 

 modified in like conditions more rapidly than the main set. Intersecting 

 fringes producing a beaded structure and fringes moving in opposite directions 

 are also observed. 



55. Character of the achromatic fringes. Since the achromatic fringes are 

 quite symmetric, consisting of a central white or fringe, flanked on either 

 side by three or four colored fringes rapidly decreasing in intensity, it is 

 obvious that (practically) they must consist of superposed monochromatic 

 confocal hyperbolas. This may be well shown in the present apparatus, 

 where the fringes are stationary and are displayed relative to horizontal and 

 vertical lines of symmetry. To carry out the experiment, it is best to insert 



