86 REPORTS ON INVESTIGATIONS AND PROJECTS. 



100-Inch Reflector. 



In Contribution No. 13 I have outlined the difficulties that must be 

 overcome in the construction and use of a lOO-inch mirror. These include : 



(i) The manufacture of a suitable glass disk. In view of their long 

 experience and full understanding of the requirements, it seems probable 

 that the St. Gobain Company will be able to make a satisfactory disk, 

 although the amount of glass to be cast in a single piece will weigh over 

 45^ tons. 



(2) The production of a perfect paraboloidal figure. After his success- 

 ful work with the 60-inch reflector, Mr. Ritchey will undoubtedly be able 

 to accomplish this difficult task. 



(3) The design and construction of a mounting capable of carrying the 

 mirror with the necessary accuracy. There seems no reason to doubt that 

 the experience gained from the use of the 60-inch reflector will render it 

 possible to design a satisfactory mounting which the Union Iron Works 

 Company will be able to construct in such a way as to meet all require- 

 ments. 



(4) Serious changes of focal length, due to variations in the tempera- 

 ture of the mirrors. The fact that the night temperature on Mount Wil- 

 son is nearly constant after 9 p. m., during the best observing season, and 

 the possibility of maintaining the mirrors during the day at the average 

 night temperature by means of a refrigerating plant, seem to indicate that 

 no insuperable difficulties will arise from this cause. 



(5) Imperfect seeing. Our tests of the definition at night on Mount 

 Wilson, made with the Snow telescope and smaller instruments, lead us 

 to believe that the occasions on which the full aperture of the mirror can 

 be used for the most exacting photographic work will not be very infre- 

 quent. The average conditions will undoubtedly permit the loo-inch 

 reflector to be used advantageously in the various classes of work in 

 which large light-gathering power, rather than the most perfect defini- 

 tion, is essential. 



Campbell, W* "W*, lyick Observatory, Mount Hamilton, California. Grants 

 Nos. 231 and 342. (a) Pay of assistants to take part in researches at the 

 Lick Observatory and iji) measurement and reduction of photographic plates 

 of Eros, (For previous reports see Year Book No. 2, p. xix ; Year Book 

 No. 3, p. 86, and Year Book No. 4, pp. 82-83.) $7,000. 



Report. — («) In the spectrographic determination of stellar velocities there 

 have been employed, as Carnegie Institution assistants, in the past year : 



Dr. Joseph H. Moore, October i, 1905, to July i, 1906. 

 Mr. Keivin Burns, October i, 1905, to October i, 1906. 

 Dr. B. Iv. Newkirk, September i, 1906, to October i, 1906. 



During ten months of work Dr. Moore secured 185 spectrograms with the 

 Mills spectrograph attached to the 36-inch refractor, made first measure- 



