MOUNT WILSON SOLAR OBSERVATORY. 20J 



CONSTRUCTION DIVISION. 



The work of construction, though still delayed in the case of the ioo-inch 

 reflector, has been active in several departments during the past year. The 

 Director, aided by Mr. Adams, has been in general charge of all construc- 

 tion, and has devoted much time to questions of design. Mr. Ritchey, since 

 his return to the Observatory on January i, after an absence of three months 

 on account of illness, has had immediate charge of the optical work on the 

 ioo-inch mirror. Mr. Drew has carried forward the design of the ioo-inch 

 mounting, while the detailed drawings of the dome have been made by 

 Messrs. D. H. Burnham & Co., of Chicago. Mr. Jones has continued his 

 supervision of building construction in Pasadena and on Mount Wilson, and 

 Mr. Ayres has remained in charge of the instrument shop. Mr. Pease, in 

 conjunction with various members of the staff, has designed a large number 

 of instruments and accessory apparatus. 



the; ioo-inch telescope. 



The long delay in the construction of the ioo-inch reflector, occasioned by 

 the difficulty of casting a suitable glass disk for the mirror, has not been 

 without some compensating advantages. The opportunity has been utilized 

 to make extensive studies of the best type of mounting and dome, and to 

 undertake various supplementary investigations, which may have an im- 

 portant bearing upon the final success of the telescope. It is evident that 

 any considerable improvement in photographic processes, such as an increase 

 in the sensitiveness of plates without a corresponding increase in the size of 

 the silver grains, would have the effect of adding to the efficiency of the 

 instrument. In the same way the improvement of auxiliary apparatus, such 

 as the optical parts of the spectrograph to be used with the telescope, would 

 mean more and better observational results. A great additional gift to the 

 Carnegie Institution of Washington by Mr. Carnegie, accompanied by an 

 expression of his hope that provision would be made for the ioo-inch tele- 

 scope, has rendered possible a general study of these and other questions 

 upon which the efficiency of the telescope depends. 



In the matter of design, some difficult questions have presented them- 

 selves. Mr. Ritchey favored the adoption of a modified Cassegrain type of 

 reflector, of a form similar to that first suggested by Schwarzschild, in which 

 a large field is obtained by the use of mirrors of special figure. For the 

 photography of nebulas and other work requiring a large field of good defi- 

 nition, such a plan is very attractive, especially in view of the short and 

 compact tube required for the mounting. But two important considerations 

 seemed decisive: It is highly desirable to be able to work in the principal 

 focus of the ioo-inch mirror, to avoid loss of light and possible distortion of 

 field and exaggerated change of focal length due to change in the figure or 

 position of the auxiliary mirror. In fact, we plan to make the greater part 



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