REVERSED AND NON-REVERSED SPECTRA. 



139 



no more so than the estimated data imply. In fact, the particular adjust- 

 ment for achromatic fringes gave a larger result r = ^2 cm. /radian, for reasons 

 not apparent. One may note that the correction for glass-path is small. 



With regard to the application to the electrometer, we may come to the 

 following conclusion: A good instrument of the quadrant or similar type 

 should give about a radian of deflection per volt, or a microradian per micro- 

 volt. In the present interferometer the microradian is about equivalent to 

 the passage of one interference fringe. Hence one fringe per microvolt is 

 about the order of sensitiveness obtainable. 



72. Separated rigid vertical system. The method used above, of attach- 

 ing the deflecting mirrors at 45 to the U-tubes, is faulty in design and dif- 

 ficult to adjust. It was, therefore, subsequently discarded in favor of the 

 rigid deflecting system, figure 96, consisting of a wide mirror at 45, m, and 



a rrS Of 

 |[ 



771 



a horizontal mirror m 1 '. Both must be provided with adjusting screws for 

 axes respectively parallel to the traces m and m' and normal to the diagram, 

 if the fringes when found are to be centered and enlarged. It is particularly 

 essential that the framework supporting m and m' be rigid, otherwise the 

 quiver introduced here is superposed on and accentuates the corresponding 

 tendency of the very mobile liquid column. This is contained in the brass 

 tube C (with glass window at g) which is supported on an independent stand- 

 ard, rigidly. There are two of these tubes (C, C', fig. 97), side by side, 

 parallel, and at the same level, one for each component beam. As the rays 

 aeh are to retrace their path, the apparatus, figure 88, should be used, with 

 the mirror m of that figure tilted up as in figure 96. Hence the two component 

 beams ae and bf, figure 88, prolonged as at eh, figure 96, each penetrate a 

 column C and are returned by the normal mirror m'. Thus one reflection 

 of the old method is obviated and adjustment is facilitated, since both beams 

 are reflected from the common mirrors m and m'. To assist in the preliminary 

 adjustment, the rigid system m, m' should be capable of revolving roughly 

 about a vertical axis. With a wide beam the rays may then be guided by 

 the eye to retrace their path. Fine adjustment is made by the triple screws 

 on m and m' already mentioned. 



