264 



SCIENCE. 



[N. S. Vol. XV. No. 372. 



gulation, or resort to comparatively inac- 

 curate star-places, such as those obtained 

 with meridian instruments. Proper motion, 

 also, which necessitates new heliometric 

 triangulations made very near the date of 

 the photographic observations, is altogether 

 eliminated in the use of this method. 



It is merely necessary to arrange the tele- 

 scope so that it can be rotated around its 

 optical axis, or some other axis parallel to its 

 optical axis. Suppose two photographs of 

 a group of stars have been made with such 

 a telescope, rotated 90° between the two 

 exposures. If, then, the object glass pos- 

 sesses the peculiarity of making all the 

 Y-coordinates too large in the first expo- 

 sure, the same peculiarity will show itself 

 in the second exposure by making all the 

 X-coordinates correspondingly too large. 

 Thiis it is sufficient to make a series of 

 negatives of the same star-group, rotating 

 the instrument through varioxis angles be- 

 tween the exposures, when a simple com- 

 parison will surely bring to light any form 

 of optical distortion depending on the 

 direction of measurement upon the plate. 



The process is a purely differential one, 

 and requires only a roughly approximate 

 knowledge of the absolute star-positions, 

 sufficient for the computation of refrac- 

 tion corrections, etc. It can be applied 

 to an equatorial telescope of the ordinary 

 form if we photograph the region imme- 

 diately su.rrounding the pole of the heavens. 

 In that case, the polar axis of the equatorial 

 becomes a suitable axis for rotating the 

 telescope, since the polar axis is parallel to 

 the optical axis, when the tube is pointed 

 at the pole. It is obvious that a trial of 

 this method Avill furnish not only a de- 

 termination of optical distortion, but will 

 yield also, as a sort of by-product, a photo- 

 graphic catalogue of the close polar stars. 

 For this reason it seemed desirable to in- 

 clude in the work a set of plates of the 

 south pole as well as the north. In this way 



we should obtain very precise catalogues 

 of both sets of close polar stars, all reduced 

 and computed according to a uniform 

 method. 



In 1895 the writer was visting at 

 the Cape of Good Hope Observatory, and 

 discussed the matter with Sir David 

 Gill. The plan met with his approval, and 

 he consented gladly to make the necessary 

 south polar plates. With equal readiness. 

 Dr. Anders Donner, of the Helsingfors Ob- 

 servatory, offered to make the north polar 

 plates. These latter negatives were meas- 

 ured at Columbia University by Mrs. Her- 

 man S. Davis and Mrs. Annie Maclear 

 Jacoby; the measures were reduced at 

 Vassar College by Miss C. E. Furness; and 

 they were published by the Vassar College 

 Observatory. The south polar plates were 

 similarly measured and reduced at Colum- 

 bia by Misses F. B. Harpham, Mary Tar- 

 box, Eudora Magill and H. L. Davis, and 

 the results will soon be published by the 

 Observatory of Columbia University. The 

 researches for both poles agree in showing 

 that the optical distortion depending on 

 direction of measurement is too small to be 

 detected with certainty even by the delicate 

 differential method here described. 



W. S. BiCHBLBERGER, 



For the Council. 

 {To be concluded.) 



THE V. S. COAST AND GEODETIC SURVEY. 



The last annual report* of the Superin- 

 tendent of the United States Coast and 

 Geodetic Survey to Congress is fully illus- 

 trated Awth maps and diagrams and pre- 

 sents in detail the work accomplished by 

 this bureau for the fiscal year ending June 

 30, 1901. 



Throughout the report there is frequent 

 evidence of the increased scope of the Sur- 

 vey's operations mthin the last few years, 

 as well as proof of the flexibility of the 



* Now in the hands of the printer. 



