EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 33 



vided with a fine motion. The clutch e and the corresponding one for M 

 (not shown) allow the micrometer to be placed at a greater or less distance 

 from the grating. The center of the mirror is about on the same horizontal 

 level as the grating. It is also usually convenient to place the lens L of the 

 collimator in its screen, on the same rod cd, with an appropriate clutch and 

 rack and pinion. 



The complementary framework is shown in fig. 21 and holds the slit 5 of 

 the collimator (or the filament of a Nernst lamp) and the two telescopes T and 

 T' in place for observation, T being used for the direct slit image and T' for 

 the diffraction spectra and interferences. The framework fghi is, as before, of 



FIG. 20. 



FIG. 21. 



gas-pipe, / being firmly plastered into the wall on the front face of the pier 

 (the other one, ab, being on the side). The telescopes T and T' are necessarily 

 adjustable on a horizontal and vertical axis, and may be raised and lowered 

 and moved right and left along the rod kl, held by a firm clutch at k. The 

 lens L may also be carried on ab, as has been stated. Right-and-left, up-and- 

 down motion is needed for the insertion of these appurtenances. The rods cd 

 and hi are not in the same horizontal or the same vertical plane, so that the 

 systems may be superposed as stated. 



In the course of the work it appeared, however, that the framework of 

 simple pipe was annoyingly subject to tremors. It was found necessary to 

 lengthen the rods gd and gc, g'i and g'h to over a meter in length. Hence it 

 was preferable to bolt the pairs of parallel rails together for increased stiffness 

 and to secure the ends of each pair with a wide tetrahedral brace of gas-pipe, 

 abutting at the pier, against horizontal and vertical displacement. So ad- 

 justed, the system was light and rigid and easily modified for the different 



