ADVISORY COMMITTEE ON ASTRONOMY 143 



reduces the diflBculty of maintaining the image of a celestial object 

 at a fixed point on the photographic plate, has proved to be of great 

 importance. With its aid Common secured excellent photographs 

 of the Great Nebula in Orion. 



The high opinion of the possibilities of the reflecting telescope 

 which is entertained by astronomers at the present time is based in 

 large degree upon the results obtained by Keeler with the Crossley 

 reflector of the I,ick Observatory and by Ritchey with the two-foot 

 reflector of the Yerkes Observatory. The Crossley reflector, with 

 three-foot mirror by Grubb, is not provided with a thoroughly 

 modern mounting.* In ordinary hands it is not improbable that 

 photographs representing no marked advance would have been ob- 

 tained with it ; but through Keeler' s skill the instrument was made 

 to yield results of unexpected excellence. Few spiral nebulae had 

 previously been known to exist, but Keeler' s photographs showed 

 them to be as numerous as all other forms combined ; indeed, they 

 probably represent the type object to which our present ideas of the 

 nebular hypothesis must be made to conform. Without the aid of 

 a large reflector in competent hands it is doubtful whether this fun- 

 damentally important discovery would ever have been made. 



Ritchey's results, though obtained with an instrument of only two 

 feet aperture, constructed in the shops of the Yerkes Observatory, 

 leave no element of doubt as to the great possibilities of the reflector. 

 With this small instrument stars too faint to be seen or photographed 

 with the 40-inch Yerkes telescope (the largest refractor hitherto 

 constructed) can be photographed in forty minutes. The faintest 

 star within reach of the 40-inch telescope is of the seventeenth mag- 

 nitude, but with the two-foot reflector photographs made with ex- 

 posures of six or seven hours show stars which are estimated to be 

 one or two magnitudes fainter. The advantages of the reflector are 

 still more striking in the case of the nebulae, especially for such 

 objects as the Great Nebula in Orion, the Great Nebula in An- 

 dromeda, the nebulae in the Pleiades, the spiral nebulae — indeed, for 

 all such objects except the minute planetary nebulae, which would 

 also be shown to advantage by a reflector of great focal length. 

 Furthermore, measurements of stellar photographs made with the 

 Crossley three-foot reflector show that if the field employed is not 

 too large the positions of stars can be determined with precision. 

 For spectrographic research the reflector offers great advantages 

 over the refractor, especially for work in the hitherto almost unex- 



* Such a mounting is about to be constructed. 



