Barus — Displacement Interferometer. 85 



slit being opened wide. The electrometer is now rotated as a 

 whole around the vertical, until the light reflected from 

 n strikes m and a similar screen behind the latter in the 

 line mn is necessary here. Next, the mirror iVis adjusted to 

 normality, a white screen behind m in the direction Jym being 

 essential, whereupon the light is reflected from n again toward 

 the grating. As the pencil must pass through the grating G 

 again, this is mounted to be capable of being raised or lowered 

 and rotated around the vertical and horizontal (adjustment 

 screws); but these are the usual adjustments on the inter- 

 ferometer. It follows that the electrometer must also be 

 capable of being raised and lowered on its long foot screws, as 

 already indicated. 



Moreover, the spectrum should be so placed by raising and 

 lowering the source of light and including the grating together 

 with N that the higher orders of direct spectra are not in the 

 same plane with the spectra superposed for interference. 

 When the adjustments described are well made, there is no 

 further difficulty in finding the solitary ellipses. As there is 

 compensation owing to. the two glass plate windows in the case 

 of the electrometer, the ellipses are liable to be enormous, 

 practically vertical straight lines, which are displaced with cor- 

 respondingly great rapidity by the micrometer screw, and are 

 therefore hard to find. Hence some counter compensation at 

 the micrometer mirror is desirable, in order that centers may 

 appear, and the displacement may be slower. A compensating 

 plate about 1 centimeter or less in 'thickness, with the vertical 

 focus of the light from the slit on the stationary mirror, pro- 

 duces very clear and sharp ellipses, admirably adapted for 

 measurement. 



17. Observations. — The stems of the mirrors m and n on rr 

 were about a — 4\5 centimeters apart. Hence, since i = 45°, a 



— (1 S 



= .4"5A/2 and if 8 is the path difference -—r = "0175 X 2<z sin 



d\ 



i = "079 cm. per degree. Supposing that 1 volt gave a 

 deflection of 45° and that 10 -4 centimeters are measurable 

 at the micrometer, the micrometer sensitiveness should be 

 10~ 4 /45 X -079, or about 3 X 10~ 5 volt, i.e., about l(r 5 volt 

 per vanishing interference ring. It would thus seem probable 

 that with a lighter needle and a more delicate suspension the 

 possibility of measuring 10 -6 volt would not be out of the 

 question. The following observations show that these surmises 

 are correct so far as the method is concerned. 



The great hardship encountered in the present work was the 

 unavoidable agitation of the laboratory, and this unfortunately 

 is insuperable. After the ellipses were found they were 

 always in motion, so that the displacement work was bound to 



