SOLAR OBSERVATORY. I7I 



1.3''. While these prehminary data are not strictly comparable, on account 

 of the different kinds of plates employed, they at least show that the 6o-inch 

 is already yielding excellent images. Their quality is best illustrated by Pro- 

 fessor Ritchey's recent photographs of nebulae, some of which will soon be 

 published. 



Through the kindness of Professor Campbell and Professor Parkhurst, 

 a series of comparative tests of the 24-inch Yerkes reflector, the 36-inch 

 Crossley reflector, and the 60-inch Mount Wilson reflector will be made in the 

 near future. 



The observing program for the 60-inch reflector is not yet definitely 

 arranged. For present purposes, the work so far accomplished may be 

 classified as follows : 



StelIvAr Photography. 



With the Newtonian arrangement of mirrors, a large number of photo- 

 graphs has been taken in the principal focus for use in connection with 

 Professor Kapteyn's studies of the stars in his Selected Areas. A number 

 of the earlier photographs were made by Dr. Abetti, but since March, 1909, 

 all of this work has been done by Mr. Babcock. The negatives obtained in- 

 clude 39 photographs of stars in the Selected Areas, 12 photographs contain- 

 ing focal and extra-focal images of clusters and comparison stars, made with 

 and without a red screen, for the determination of the absorption of light in 

 space; 5 photographs of stars for the determination of the absorption in 

 space, taken with and without a red screen; 12 parallax plates, each having 

 two exposures ; 8 photographs of stars at the pole ; and a number of miscel- 

 laneous plates for various purposes. 



Parallax Investigations. 

 In order to gain some idea of the precision obtainable in parallax determi- 

 nations made by the method of Kapteyn with the 60-inch reflector, a series 

 of test plates was made in the Newtonian focus of this instrument by Mr. 

 Babcock. Five of these plates were measured by Miss Ware, and from a 

 discussion of the results by Professor Scares it seems likely that parallaxes 

 can be determined with the 60-inch reflector with a probable error not ex- 

 ceeding o'^oi6. There is every reason to believe that the use of the lOO-foot 

 focus combination will afford results of an even higher degree of precision. 



Photography of Nebula and Star Clusters. 

 Professor Ritchey has devoted all of his nights with the 60-inch reflector 

 to the photography of nebulae and star clusters, both in the principal focus 

 and with the Cassegrainian combination having an equivalent focal length of 

 100 feet. Much time has necessarily been spent in experimenting, in order to^ 

 obtain the best means of guiding and focusing and of eliminating difficulties 

 arising from changes of focal length of the telescope during the exposure. 



