518 



SCIENCE 



[N. S. Vol. XLV. No. 1169 



photography of nebulie in the '90 's added 

 to o-iir knowledge of them, but the number 

 of spirals known to exist was still not more 

 than two or three dozen out of the approxi- 

 mately 10,000 objects listed in Dreyer's 

 General Catalogues of nebula? and clusters. 



discovery that the great majority of the 

 nebula? are spirals. Keeler's resiilts have 

 been abundantly confirmed by later ob- 

 servers: Perrine, Wolf, Fath, Curtis and 

 others. Using the sixty-inch reflector of 

 the Jilount Wilson Solar Observatory, Fath 

 photographed 139 small areas uniformly 

 distributed over the northern three fifth.s 

 of the sky, and he estimated from the num- 

 ber of the nebulffi recorded on his small 

 plates that similar photographs completely 

 covering the sky. 22.000 photograplis. would 



Fig. 4. Spiral Nebula Messier 81, photographed 

 by Keeler with the Crossley Reflector of the Lick 

 Observatory. 



A great impetus was given to the study 

 of the nebulffi when Keeler began to photo- 

 graph them, in 1898, with the Crossley re- 

 flecting telescope of the Lick Observatory. 

 His photographs, covering small areas in 

 many different parts of the sky, recorded 

 hundreds of nebula? hitherto unseen. Using 

 these small photographs as sample tests of 

 nebular distribution, he estimated con- 

 servatively that his telescope could discover 

 at least 120,000 nebulce in the entire sky 

 whenever he cared to undertake the work. 

 Keeler's photographs were revolutionary in 

 another sense: they led him to the capital 



Fig. 5. Spiral Nebula Messier 51, photographed 

 by Curtis with the Crossley Reflector of the Lick 

 Observatory. 



record about 162,000 nebula?. Perrine, 

 using the Crossley reflector, estimated the 

 number discoverable as still greater. All 

 the observers have found the faint nebula 

 to present a variety of elliptic forms, such 

 as we should expect if they are spirals 



