176 REPORTS ON INVESTIGATIONS AND PROJECTS. 



telescope is now essentially complete. The concrete laboratory at its base 

 has also been finished and the mirrors and 150- foot focus lens are installed. 



The general features of the telescope were described in the last report. 

 The erection of the two towers, one inclosed within the members of the other, 

 has been carried out most successfully by Mr. Jones. The dome which 

 covers the instruments at the top of the tower consists of two approximately 

 hemispherical sections of slightly different radius, one of which may be 

 rotated within the other. This form of construction has the marked advan- 

 tage of affording a very large opening and thus promoting the free circula- 

 tion of air about the mirrors. A square tube 5.5 feet in size, built of angle 

 iron and lined with sheet iron, extends the full height of the tower, inclosing 

 the beam of light from the lens and protecting it from disturbing air-currents. 

 To prevent heating, the outside of the tube is covered with canvas louvers of 

 a form similar to those used on the Snow telescope house. The dome is 

 supported by the outer tower, and the tube jointly by the outer tower and the 

 concrete roof of the laboratory. 



The mirrors which have been figured in the Pasadena optical shop are 

 about 12 inches thick and similar to those which have proved so successful 

 in use with the 60-foot tower telescope. To prevent distortion due to heat- 

 ing of the edges, the sides of the mirrors are incased in closely fitting water- 

 jackets through which a stream of water may be kept circulating. The lens 

 now in use is 12 inches in diameter and of the triple construction designed by 

 Hastings to reduce the effect of chromatic aberration. The 75-foot focus 

 lens for use in the spectrograph is of the same form. Both lenses will be 

 given a careful test during the next few months. 



The few preliminary trials thus far made with the tower telescope indicate 

 that it should prove most successful in regular operation. The 17-inch 

 image of the sun shows excellent definition on days when the observing con- 

 ditions are good, and there appears to be an entire absence of vibration due 

 to shaking of the tower by the wind. 



THE MONASTERY. 



On December 13, 1909, a fire caused by an overheated range entirely de- 

 stroyed the Monastery. A considerable number of valuable books were 

 burned, but fortunately little was lost that can not be replaced readily. As 

 soon as the rainy season was over the work of rebuilding was commenced, 

 and is now completed. The present building is of reinforced concrete 

 throughout and follows closely the general design of the former Monastery. 

 A few additional rooms have been provided and more convenient heating 

 and water systems installed. The Observatory is indebted to the Carnegie 

 Institution of Washington for a special appropriation by means of which it 

 was enabled to reconstruct so promptly and in such a satisfactory and per- 

 manent form the building about which so large a part of the social life on 

 Mount Wilson naturally centers. 



