H. Crew — Rotation of the Sun. 151 



In regard, then, to the Lippmann's Electrometer it has been 

 shown : First, that, when hydrogen polarization is used, the 

 deflections of the meniscus may be taken as proportional to the 

 E. M. F. for very low potentials, and that for potentials up to 

 09 D. an empirical curve will show the relation between the 

 E. M. F. and the deflection. Secondly, that polarization is 

 complete, and that no appreciable current passes through the 

 electrometer until it be charged to a potential near that at 

 which electrolytic action begins. Thirdly, that the capacity of 

 the Lippmann Electrometer is very considerable, compared, for 

 example, with that of the Thomson Quadrant Electrometer, 

 being in the particular instrument studied, several hundred 

 microfarads. 



The investigation, of which a summary is given in the pre- 

 ceding pages, was carried on at the Sloane Physical Laboratory 

 of Yale University under the direction of Prof. A. W. 

 Wright, to whom I would here express my grateful acknowl- 

 edgment for his encouragement and advice. 



Cornell University, Ithaca, N. T., November, 1887. 



Art. XIII. — On the Period of the Rotation of the Sun as 

 determined by the Spectroscope • by Henry Crew, Assistant 

 in Physics, Johns Hopkins University. 



Zolljster* and Vogel were the first to measure the rotation 

 of the sun by the use of Doppler's principle. For this pur- 

 pose, they employed the so-called "reversion spectroscope" 

 of the former. This had two prisms with their refracting 

 edges turned in opposite directions, thus forming two spectra, 

 side by side, in opposite directions, the one serving as a vernier 

 for the other; any displacement of a line will be doubled, 

 since the deviation in one spectrum is the opposite of that in 

 the other. For their results, however, the authors claim little 

 more than a qualitative value. 



Hastings, f two years later, by a very ingenious device com- 

 pared the spectra of the center and the limb of the sun, but 

 gave no quantitative observations on the displacement of the 

 lines which he observed in passing from one of these regions to 

 the other. Langley:}; has devised an instrument which gives, in 

 juxtaposition, the spectra of light from any two points, distant 

 180° on the circumference of the solar disc. He has noted, in 

 rather an incidental way, the displacement due to rotation of 



* Zolluer: Pogg. Ann., cxiv, 449, 1871. 

 f Hastings: this Journal, v, 369, 1873. 

 % Langley: this Journal, xiv, 140, 1877. 



