SCIENCE. 



[N. S. Vol. VIII. No. 195. 



THE DEVEL0P3IENT OF PHOTOQBAPHY IN 

 ASTRONOMY (11.).* 



The great nebula of Andromeda has 

 been known to astronomers for upwards 

 of a thousand years, as it is a rather 

 noticeable object with the naked eye. 

 It has received its full share of atten- 

 tion ever since the invention of the tele- 

 scope. It also appears to be a vast body of 

 gaseous matter shining far away in the 

 depths of space, but it seems to be in a dif- 

 ferent condition from that of the great 

 nebula of Orion, for its spectrum is contin- 

 uous and does not show the bright lines, 

 indicating an incandescent gas, which are 

 present in the spectrum of the Orion neb- 

 ula. Though the mystery of its physical 

 condition has not yet been solved, photog- 

 raphy has at least shown us its true form. 

 It has ceased to be the long spindle-shaped 

 body of the elder Herschel, or the broad 

 irregular object, with two dark parallel 

 channels in one side, as drawn by the 

 Bonds and Trouvelot. The photographic 

 plate reveals to us in their place a beautiful 

 symmetrical mass of nebulous matter, sur- 

 rounded with several more or less concen- 

 tric rings, claimed by some astronomers to 

 be a representation of the nebular hypoth- 

 esis of La Place in full operation. The 

 first picture to show the true form of this 

 wonderful object was taken in 1S85, with a 

 20-inch reflector, by Eoberts. It does not, 

 however, require a powerful photographic 

 telescope to show its peculiar features, for 

 a 6-inch portrait lens will show the rings 

 well with anything above one hour's ex- 

 posure. 



Important photographs of some of the 

 spiral nebuliB, and especially of the ' whirl- 

 pool nebula ' of Lord Rosse, were made 

 as early as 1888, by von Gothard, with a 

 10-inch reflector. Excellent photographs 



*Address o£ the Vice-President before Section A — 

 Mathematics and Astronomy — of the American Associ- 

 ation for the Advancement of Science, August 22, 1898. 



of these objects have also been obtained by 

 Dr. Eoberts. 



While it is absolutely necessary to use a 

 considerable photographic telescope for the 

 accurate registration of star positions, etc., 

 where measures of precision are required, 

 there are a great number of objects in the sky 

 which are not necessarily subject to meas- 

 urement, and which for their greatest value 

 require a simple pictorial representation. 

 The Milky Wajs one of the most beautiful 

 and certainly the most stupendous of the 

 celestial features, is not susceptible of ac- 

 curate measurement. The individualizing 

 and measurement of all its stars would be 

 the most hopeless task imaginable. Nor 

 would such a task be of any very great im- 

 portance could it be accomplished as a 

 whole, for we could not form any special 

 idea of its sti-uctural peculiarities from such 

 a work. Though a conspicuous object to 

 the unaided eye, the view we thus get of it 

 is not sufficiently tangible, from the lack of 

 details, to enable one to form any more 

 than a crude idea of its coarser features. 

 But even the naked-eye view of it is far 

 more comprehensive than a catalogue would 

 be containing accurate determinations of all 

 its individual stars. "What is required, 

 therefore, in the study of this wonderful 

 object — this mighty universe of stars — is 

 something that will increase the penetra- 

 tion of our vision, and at the same time 

 give us a certain amount .of accuracy of 

 position with a large field of view, so that 

 we may study its peculiarities of structure 

 in detail, and at the same time closely 

 locate these details with reference to the 

 whole, and thus, by finally putting structure 

 and detail together, form a comprehensive 

 idea, not only of the details themselves, but 

 also of the relation of these features to each 

 other. The long-focus telescope with a very 

 limited field is not capable of dealing with 

 the Milky Way in the manner stated. Its 

 structural details are very large, far larger 



