MOUNT WILSON OBSERVATORY. 217 



ONE-HUNDRED-INCH TELESCOPE. 



Preliminary tests of the 100-inch telescope, arranged in the Casse- 

 grain form, were secured early in November 1917 under moderately 

 favorable conditions of observation. A number of objects, including 

 the Moon, Saturn, and several stars were examined, and the tests 

 showed conclusively that the great light-gathering power of the instru- 

 ment will be used to full advantage under good observing conditions. 

 An extraordinary amount of minute detail was seen in the case of the 

 moon. The images, however, showed considerable flare which appeared 

 to be due in part to temperature effects, and in part to imperfect 

 collimation resulting from lack of rigidity in the support of the convex 

 mirror of the Cassegrain combination. In order to obviate the latter 

 difficulty a compression ring has been added to the tube section carry- 

 ing this mirror. This affords ample rigidity, and the temperature 

 control for the 100-inch mirror is now being completed. A periodic 

 error of small amplitude and short period was observed in the driving 

 of the telescope and is now under investigation. A most satisfactory 

 feature of the telescope is the ample power of the driving-clock and 

 the ease of control of the entire instrument. 



The observing-platform has been partially reconstructed during the 

 year, with a view to reducing its weight and aiding the convenience 

 of operation. Essentially all of the smaller parts of the telescope 

 have been finished and assembled and the electrical installation is 

 now complete. It is our plan to use the telescope first in the Casse- 

 grain form in conjunction with the spectrograph and plate-holder 

 at the secondary focus. As in several other branches of construction 

 work, the progress of this instrument has been greatly delayed by the 

 large amount of time devoted in the machine-shop to activities arising 

 from the war. 



CONSTRUCTION WORK ON MOUNT WILSON. 



The greater part of the construction work on Mount Wilson has 

 been of a minor character and has consisted in repairs and improve- 

 ments of the buildings and grounds. A considerable amount of labor 

 has been devoted to the clearing of fire-breaks and the elimination of 

 the more inflammable material surrounding the buildings. In addition 

 reference may be made to the remodeling of the Hooker cottage; the 

 building of an approach to the 100-inch telescope dome; an extension 

 of the steel roof of the power-house ; repairs of the roof of the Monas- 

 tery; and much miscellaneous painting, including the dome of the 

 large reflector. 



The construction of the high-tension transmission line by the South- 

 ern California Edison Company was completed during the autumn 

 of 1917, and since that time regular use has been made of the electrical 

 energy supplied by this line, our own power-plant being employed 



