October 26, 1900.] 



SCIENCE. 



631 



revealed ; on one plate no less than thirty- 

 one new ones were found. The accurate 

 positions of these new nebulse are now being 

 measured by Mr. Palmer. He finds, on the 

 average, about ten new nebulte to the plate. 



Professor Keeler summarizes the con- 

 clusions to be drawn from the work so far 

 accomplished as follows : 



" 1. Many thousands of unrecorded neb- 

 ulse exist in the sky. A conservative es- 

 timate places the number within reach of 

 the Crossley reflector at about 120,000. The 

 number of nebulse in our catalogues is but 

 a small fraction of this. 



" 2. These nebulse exhibit all gradations 

 of apparent size, from the great nebula in 

 Andromeda down to an object which is 

 hardly distinguishable from a faint star 

 disk. 



" 3. Most of these nebulse have a spiral 

 structure. 



" To these conclusions I may add another, 

 of more restricted significance, though the 

 evidence in favor of it is not yet complete. 

 Among the objects which have been photo- 

 graphed with the Crossley telescope are 

 most of the ' double ' nebulse figured in Sir 

 John Herschel's catalogue (Phil. Trans., 

 1833, Plate XV.). The actual nebulse, as 

 photographed, have almost no resemblance 

 to the figures. They are, in fact, spirals, 

 sometimes of very beautiful and complex 

 structure ; and, in any one of the nebulse, 

 the secondary nucleus of Herschel's figure 

 is either a part of the spiral approaching 

 the main nucleus in brightness, or it can- 

 not be identified with any real part of the 

 object. The significance of this somewhat 

 destructive conclusion lies in the fact that 

 these figures of Herschel have sometimes 

 been regarded as furnishing analogies for 

 the figures which Poincar6 has deduced, 

 from theoretical considerations, as being 

 among the possible forms assumed by a ro- 

 tating fluid mass ; in other words, they have 

 been regarded as illustrating an early stage 



in the development of double star systems. 

 The actual conditions of motion in these 

 particular nebulse, as indicated by the 

 photographs, are obviously very much more 

 complicated than those considered in the 

 theoretical discussion." 



As evidence of the power of the Crossley 

 telescope it may be noted that a very faint 

 image of the King ISTebula in Lyra was 

 obtained with an exposure of thirty sec- 

 onds ; with an exposure of two minutes a 

 well marked impression of the nebula is 

 obtained and a surprisingly strong image 

 of the central star, which is a very faint 

 object visually in the 36-inch refractor. 



In the course of the work on the nebulse, 

 two new asteroids have been discovered, by 

 means of their trails, one at least of which 

 was so faint as not to be seen with certainty 

 with the large refractor. Observations of 

 these asteroids were made photographically, 

 and were found to compare very favorably 

 in accuracy with such observations made 

 visually with a large refractor. These re- 

 sults point to the great value of this 

 instrument for finding and giving the 

 positions of asteriods, whose places are 

 approximately known. 



One of the most promising fields for the 

 Crossley reflector is undoubtedly that of 

 stellar spectroscopy. Two spectrographs 

 have been designed and built at the obser- 

 vatory. One is due to the generosity of 

 Miss C. W. Bruce, and contains three 

 prisms of 60° and one of 30°, with an aper- 

 ture of two inches ; the other has a single 

 quartz prism and is intended to give meas- 

 urable, though small, spectra of some spe- 

 cial objects nearly at the limit of vision of 

 the telescope. 



From what has been said, it will be seen 

 that a large amount of work of great impor- 

 tance has already been accomplished with 

 the Crossley reflector, besides opening up 

 new fields for future investigation. 



Professor Keeler clearly recognized the 



