MOUNT WILSON OBSERVATORY. . 263 



134-foot focus, but on account of the pressure of other work it has not 

 yet been feasible to make simultaneous exposures on the same cluster 

 with the 60-inch under good conditions of seeing. The large scale of 

 the Hooker photographs will permit the magnitudes of stars in the 

 central parts of clusters to be determined with less disturbance from 

 the Eberhard effect than has been experienced with the 60-inch. 

 Simultaneous cluster photographs made with the two telescopes when 

 the seeing was very poor (1 on a scale of 10) show decidedly better 

 results for the 60-inch, as would of course be expected from its much 

 shorter focal length. 



Photographs of the moon have been made at the 134-foot focus of 

 the Hooker telescope by Mr. Pease, with very satisfactory results. 

 Many of these appear to be decidedly superior in definition to any 

 previously taken with other instruments. The extraordinarily minute 

 structure seen visually on many occasions indicates that under the 

 best atmospheric conditions still better photographs can probably be 

 obtained. 



Some interesting photographs of Campbell's star -with hydrogen 

 atmosphere (B D + 30°3639), showing a curious rift in the atmos- 

 phere on one side, have also been made by Mr. Pease at the 134-foot 

 focus, as well as several photographs of very small planetary nebulae 

 showing minute details of structure. Mr. Pease's photographs of 

 Campbell's star with a sUtless spectrograph seem to show that it is an 

 annular nebula. 



Some interesting experiments by Mr. Shapley and Mr. Benioff at 

 the 134-foot focus of the Hooker telescope indicate important possi- 

 bilities in photographing very faint stars. By placing a lens immedi- 

 ately in front of the photographic plate in such a way as to make the 

 rays more rapidly convergent, thus bringing the equivalent focal length 

 down to about 30 feet, stars have been photographed which are about 

 2.5 magnitudes fainter than those obtained during an equal exposure 

 without the lens. Such a device is certain to have important appli- 

 cations, and it remains to be seen what a similar arrangement \\dll 

 accomphsh at the principal focus of the Hooker telescope, where the 

 star images will naturally be much smaller. 



Some promising preliminary work (not using the lens just mentioned) 

 has been done with improvised arrangements in the principal focus of 

 the 100-inch mirror, but definitive tests at this point must await the 

 completion of the Newtonian cage and the observing-platform, which 

 will soon be ready for use. 



CONSTRUCTION WORK ON MOUNT WILSON. 



Most of the construction work of the year has been done in Pasadena, 

 where a two-story building about 46 by 90 feet in size, together with a 

 storehouse and furnace-room, were erected and equipped in connection 



