l80 REPORTS ON INVESTIGATIONS AND PROJECTS. 



The construction of a mounting for a concave grating of i-meter radius. 



The construction of a water-distilling apparatus for use with the exhaust 

 of the gas-engine in the Mount Wilson power-plant. 



The construction of the hoisting device and the figuring of various grind- 

 ing tools for use in the optical work upon the i co-inch reflector. 



In addition many minor repairs have been made upon existing instruments. 



THE 150-FOOT TOWER TELESCOPE. 



At the time at which the last annual report was written the two lenses for 

 use in the tower telescope had just been received. They were of triple con- 

 struction and designed to reduce the effect of chromatic aberration to a mini- 

 mum. A careful examination of these lenses in the optical testing-rooms in 

 Pasadena indicated the presence of strong astigmatism and considerable 

 spherical aberration, and accordingly they were returned to their makers 

 for additional figuring. On further examination it was found that one of 

 the components in each lens had indices of refraction unsuited for the pur- 

 pose in view, and the attempt was made to procure new disks of glass of 

 satisfactory indices. After a number of eft"orts in this direction without 

 success, it was decided to give up the triple form of construction and resort 

 to the ordinary doublet type. New lenses of this sort corrected visually were 

 received in July of this year and tests showed a marked improvement over 

 those tried previously. The 8-inch lens for use in the spectrograph has been 

 accepted, but the image-forming objective of 12 inches aperture is still found 

 to be subject to considerable zonal errors and has been returned to the 

 manufacturers for further retouching. 



With the exception of the image- forming lens, the 150- foot tower is now 

 complete in all of its essential features, and in case of serious delay in the 

 completion of the 150- foot objective, the lens now in use with the 60- foot 

 tower will be employed in order to gain the advantages of the more powerful 

 spectroscopic apparatus of the larger tower. 



GENERAL CONSTRUCTION WORK ON MOUNT WILSON. 



The plan of construction work on Mount Wilson during the summer has 

 been influenced to a great extent by considerations relating to the 100-inch 

 telescope. In order to provide for the concentration of building operations 

 during the next few years around the great task of the erection of the 100- 

 inch telescope mounting, it has seemed desirable to complete, in so far as 

 possible during this year, all repairs and additions to the existing buildings 

 of the Observatory, and to arrange for the necessary increase in equipment 

 which the large instrument will require. As a result the building operations 

 of the year have been of a very varied character. Among them I may refer 

 to the following: 



The Snow telescope house has been rendered essentially fire-proof, all 

 canvas having been replaced by sheet iron, and the w^ooden roof replaced by 



