INTERFEROMETRY OF SPECTRA. 85 



N. To bring out the interferences, a direct -vision prism grating g is placed 

 in front of the objective of T, whereupon, when the path-difference HM, HN 

 is annulled, magnificent ellipses may be seen in the bright spectrum in the 

 field of the telescope. 



The steel or other tube whose elongation under pressure is to be measured 

 is shown diagrammatically at / 1' . The end t', moreover, is closed by a tinned- 

 steel plug-screw, while t communicates, by aid of a thick- walled /^-inch tube 

 p of small bore, with the screw-compressor P described in my earlier papers 

 (/. c.}. It is here that the thick hydrocarbon oil is forced into the tube 1 1' 

 and the pressure measured by a Bourdon pressure-gage G, reading in steps 

 of 10 atmospheres to 1,000 atmospheres. 



The parts of the interferometer are attached directly or indirectly to a 

 brick pier in the laboratory. M is separately so attached; so is also the end 

 / of the steel tube by the bracket at B, this being fixed rigidly. The other 

 end, t', which must be free to expand, is to be supported on knife-edges or 

 rollers of a vertical pendulum hanger y, the supports of which are in turn 

 rigidly fixed to the pier. This will presently be further described. It was 

 found that long vertical wires, supporting intermediate parts of the tube, 

 were desirable and quite as good as more complicated arrangements. 



With the tube / 1' thus fixed except as to linear expansion toward the 

 right, the mirror N is clamped by a horizontal lateral arm at the end /', and 

 the half-silvered plate H by a similar arm on the other side, at the end t. 

 Thus the length HN varies with the pressure and the increment is compen- 

 sated at M by bringing the center of ellipses back to the D lines in the field 

 of the telescope. Accessories like water-jackets, etc., are left out of the fig- 

 ure for clearness and will not be used in these experiments. 



To obviate friction, the end t' of the tube 1 1' was suspended from a rec- 

 tangular yoke or pendulum consisting of two vertical rods y and y', figure 

 55, and horizontal smooth round brass cross-rods r and r' . The latter roll on 

 the round horizontal rods a and b suitably anchored at the same level in the 

 pier. The rod r' supported the free end t' of the pressure tube. 



To counteract vibrations the rod y carries a bell-shaped damper, d, below, 

 submerged in oil in the cup c. The middle of the tube /' is similarly damped 

 at its center by a bell-shaped damper in oil (not shown), against lateral and 

 vertical vibrations. 



42. Remarks on the displacement interferometer. As a rule the ellipses are 

 not seen at their best in the principal focus. The ocular must be drawn out 

 somewhat to focus them sharply; but usually the sodium lines are still visible 

 for guidance. No doubt this is due to the fact that ordinary glass plate was 

 employed at M, N, and H, figure 54, or that H was not optically plane parallel. 

 Moreover, as M is displaced, the focus of fringes changes; but as the centers 

 of ellipses are used in measurement this is no particular disadvantage. 



A few trials were made with lens compensators, but the available combi- 

 nations reduced the ellipses to horizontal sharp spindles or lines, without 



