658 POPULAR SCIENCE MONTHLY. 



well as that of Briinnow and Ball, in applying the equatorial telescope 

 to the same purposes, the parallax of nearly 100 stars has been 

 measured with some approach to precision. 



A rival method to that of the heliometer has been discovered in 

 the photographic telescope. The plan of this instrument, and its appli- 

 cation to such purposes as this, are extremely simple. We point a 

 telescope at a star and set the clock-work going, so that the telescope 

 shall remain pointed as exactly as possible in the direction of the star. 

 We place a sensitized plate in the focus and leave it long enough to 

 form an image both of the particular star in view and of all the stars 

 around it. The plate being developed, we have a permanent record 

 of the relative positions of the stars which can be measured with a 

 suitable instrument at the observer's leisure. The advantage of the 

 method consists in the great number of stars which may be examined 

 for parallax, and in the rapidity with which the work can be done. 



The earliest photographs which have been utilized in this way are 

 those made by Eutherfurd in New York during the years 1860 to 1875. 

 The plates taken by him have been measured and discussed principally 

 by Eees and Jacoby, of Columbia University. Before their work was 

 done, however, Pritchard, of Oxford, applied the method and published 

 results in the case of a number of stars. 



One of the pressing wants of astronomy at the present time is a 

 parallactic survey of the heavens for the purpose of discovering all the 

 stars whose parallax exceeds some definable limit, say 0"1. Such a 

 survey is possible by photography, and by that only. A commence- 

 ment, which may serve as an example of one way of conducting the 

 survey, has been made by Kapteyn on photographic negatives taken by 

 Donner at Helsingf ors. 



These plates cover a square in the Milky Way about two degrees 

 on the side, extending from 35° 50' in declination to 36° 50', and from 

 20h. lm. in R. A. to 20h. 10m. 24s. Three plates were used, on each 

 of which the image of each star is formed twelve times. Three of the 

 twelve impressions were made at the epoch of maximum parallactic 

 displacement, six at the minimum six months later, and three at the 

 following maximum. The parallaxes found on the plates can only be 

 relative to the general mean of all the other stars, and must therefore 

 be negative as often as positive. The following positive parallaxes, 

 amounting to 0"1, came out with some consistency from the measures: 



Star, B. D., 3972 Mag. 8.6 R. A. 20h., 2m. 0s. Dec. -f 35°.5 Par. -f 0".ll 



Star, B. D., 3883 Mag. 7.1 R. A. 20h., 2m. 3s. Dec. +36°.l Par. -j-0".18 



Star, B. D., 4003 Mag. 9.2 R. A, 20h., 4m. 58a. Dec. -j-35°.4 Par. -J-0M0 



Star, B. D., 3959 Mag. 7.0 R. A., 20h., 9m. 14s. Dec. -)-36°.3 i Par.-j-0".10 



