114 CARNEGIE INSTITUTION 



Instead of observing in an expeditious way, according to the 

 methods recommended by the Astronomische Gesellschaft, 60,000 

 stars, requiring, according to your estimate, about 150,000 observa- 

 tions, I would prefer that the positions of 30,000 only should be 

 determined with greater accuracy. This would possess the double 

 advantage of giving, on the one hand, a very precise basis for the 

 photographic catalogue of this part of the sky, and, on the other 

 hand, of providing a solid foundation for determination of the proper 

 motion of the stars. It is very desirable that we should have at our 

 disposal, at an early day, the results for this element, which is of 

 such fundamental importance in modern astronomy. With this 

 object in view, to make observations with twice the degree of accu- 

 racy is to reduce by one half the interval of time necessary to render 

 sensible the proper motions of the stars. 



It would also be desirable, in order to place the catalogue on a 

 sure and homogeneous foundation, to combine it with the 6,000 stars 

 above the seventh magnitude which you propose to determine with 

 the most rigorous precision. A catalogue constructed under these 

 conditions would certainly serve in the future as the starting point 

 for all researches of high precision concerning the study of proper 

 motions between — 32 and — 90 . 



Before leaving this subject I will present another example to illus- 

 trate its great value. In addition to the plates of short exposure 

 intended for the construction of the Astrographic Chart, comprising 

 all stars down to the eleventh magnitude, it is also intended at the 

 same time to secure plates of long exposure, guarded by the same 

 precautions and including stellar images down to the fourteenth 

 magnitude. Then, if necessary, the plates of this second series 

 could be measured and reduced with the same degree of precision as 

 the first. Since a great many of the plates contain the images of 

 1,000 to 12,000 stars, one can readily see that, after photographing 

 for a second time one of these regions so rich in stars, the compar- 

 ison would reveal a very large number of proper motions for all 

 grades of magnitude down to the fourteenth, a feature of this plan 

 which might prove very instructive from the cosmogonic point of 

 view. 



In your program you point out the necessity of determining the 

 parallax of stars; the value of such work is incontestable. 



In my opinion the surest method and the one least subject to 

 systematic error is the photographic method properly applied. It 

 would therefore be very judicious to provide for this purpose a 



