April io, 19 19] 



NATURE 



107 



quickly to within less than 2' of the catalogue 

 position, and identification is much Simplified and 

 charting- becomes unnecessary generally for any 

 stars brighter than, say, 7*5 magnitude. The 

 following given by the clock is remarkably smooth 

 and accurate, without a trace of any periodic 

 or other drift in the image, even with the great 

 focal length of io8 ft. ; the guiding for the 

 spectrograph is hence very easy. The slit of the 

 spectrograph subtends angular dimensions 3" by 

 0-3", and so accurate is the driving, and so small 

 and sharp the image in good average seeing con- 



KiG. 3.— The telescope from the south-west. 



ditions, that, unless the clock is set to drive slightly 

 fast or slow, so that the image drifts slowly from 

 one end to the other of the slit, the star spectrum 

 would be too narrow or too unevenly exposed to 

 be measurable. 



Although the telescope can be easily operated 

 by one person, ordinarily the observer is assisted 

 by the night engineer, and it is a sufficient com- 

 mentary on the perfection of the design and con- 

 struction, and on the smoothness, ease, and 

 accuracy of operation, to state that the average 

 time required to change from star to star in 

 NO. 2580, VOL. 103] 



making spectra, the time from the end of one 

 exposure to the beginning of the next, is less than 

 three minutes, and if the stars are not far sepa- 

 rated in the sky, frequently only two minutes. 

 When a single person is operating, these times 

 are increased about 50 per cent., and I do 

 not believe, notwithstanding the 45 tons weight 

 of moving parts of this telescope, that one of one- 

 fifth the aperture is generally handled so expediti- 

 ously. 



I should not be doing what is right and just if 

 I failed to express my appreciation of the suc- 

 cessful efforts of the builders of 

 this telescope to make an instru- 

 ment unequalled in quality, accu- 

 racy, and convenience. The 

 John A. Brashear Co. for the 

 optical parts, and the Warner 

 and Swasey Co. for the mount- 

 ing, have undoubtedly added 

 materially to their already high 

 reputation by their marked suc- 

 cess in this great instrument, 

 and my gratitude and that of all 

 interested in the progress of 

 science is due to them for the 

 spirit in which they attacked the 

 problems that confronted them, 

 and by their refusal to be satis- 

 fied, no matter what the cost, 

 bv anything but the best pos- 

 sible. 



.\s previously intimated, the 

 greater part of the observing 

 time of the telescope since com- 

 pletion has been occupied in 

 making star spectra, 1186 spectra 

 of stars, on the average fainter 

 than the 6th magnitude, having 

 been obtained by December 31, 

 1918. As a by-product in the 

 measurement of some of these 

 spectra, thirty spectroscopic 

 binaries have been discovered. 

 The present sjiectroscopic observ- 

 ing programme, arranged in co- 

 operation with Mt. Wilson, con- 

 sists of about 800 stars from 

 Boss's "Preliminary General 

 Catalogue," the purpose being 

 to determine the radial velocity 

 of all the stars in the catalogue 

 not previously observed and 

 within reach at the observatory and at Mt. 

 Wilson in the shortest possible time. In 

 addition, considerable time has been spent 

 on a piece of work for the late Prof. 

 Pickering-, of Harvard, obtaining direct photo- 

 graphs of the Harvard regions with and 

 without a parallel wire grating for the purpose 

 of extending the magnitude scale in these regions 

 to the faintest possible stars. Other work will, 

 of course, develop as time goes on, but in the 

 meantime, and considering the small staff—only 

 the director and Dr. Young being at present avail- 



