432 SCIENCE PROGRESS 



sisting in all of only fifty-five glass plates, each lo in. x 12 in., 

 the whole weight of which is only about 30 lb., and the price $15 

 (rather under cost, owing to the liberality of the Observatory). 

 When we compare with this the 22,154 plates of the Great Star 

 Map, weighing 3 tons, or the 2 tons of paper which the chart 

 reproductions will represent, it is clear that the discrepancy 

 needs explanation. The explanation is simply that the " Harvard 

 Sky," as it is called, though it will tell us many things, will 

 not allow us to study the changes of the stars in position, because 

 the scale is too small. Other changes can be studied with its 

 aid : thus the magnitudes of the stars are shown by it rather better 

 than on the Great Star Map, being more uniform in different 

 parts of the plate ; and we can study changes in magnitude 

 by comparing two plates taken at different times. The Harvard 

 Sky is actually being used in this way to discover new variable 

 stars, and a great many discoveries have already been made. 

 A positive copy of one plate is superposed on a negative of 

 the same region taken on a different date, and the sharp eyes 

 of three experienced ladies detect any want of correspondence 

 between the pairs of images. At the end of the year 1909 the 

 examination of twenty-one out of the fifty-five portions of the 

 whole sky had yielded 211 new variables; and the efficiency 

 of the search, and of the plates of the Harvard Sky as a 

 means of conducting the research, is constantly attested by the 

 rediscovery of known variables in the course of the examina- 

 tion. If changes in brightness of the stars were all that need 

 concern us we need have no map larger in scale than the Harvard 

 Sky, though it would be profitable to use a more powerful 

 instrument so as to show fainter stars. 



But it is important, it is indeed of the very greatest import- 

 ance, to measure also changes in the positions of the stars ; 

 for this purpose the Harvard Sky is unsuitable because of its 

 small scale. The places are correct so far as they go, but the 

 residual uncertainty is more than ten times that of the Great 

 Star Map. Consequently, to measure any given change of 

 position we should have to wait at least ten times as long, 

 and since the majority of the changes with which we are con- 

 cerned may be expected to require a century or more for 

 their complete identification on the plates of the Map as planned 

 in 1887, those who made the plans cannot be accused of extrava- 

 gance or hurry. 



