214 



POPULAR SCIENCE MONTHLY. 



SCIENTIFIC LITEKATUKE. 



PHOTOGRAPHY OF SOLAR 



ECLIPSES. 

 It is often supposed by readers of 

 popular articles on astronomical pho- 

 tography that the introduction of the 

 methods of 'the new astronomy' has 

 done away, once for all, with the diffi- 

 culties of the old. The photographic 

 plate has taken the place of the observ- 

 er's eye and the personal equation is 

 supposed to have been abolished. Those 

 who work in astronomical photography 

 are the first to extol the merits of the 

 new methods. But they are fully aware 

 of difficulties peculiar to them which 

 must be treated very much as if they 

 were errors peculiar to an observer. The 

 plate has its own personal equation. It 

 is impossible to overestimate the bene- 

 fit to eclipse observations, for example, 

 that has resulted from the introduction 

 of photography as a means of register- 

 ing the forms and details of the solar 

 corona. Yet the photographic plate has 

 serious failings of its own. Some of them 

 have lately been done away with by a 

 device invented by Mr. Charles Burck- 

 halter, Director of the Chabot Observa- 

 tory, in Oakland, California; and it is 

 the purpose of this paragraph to exhibit 

 the advance made by Mr. Burckhalter's 

 methods. 



The solar corona is very bright near 

 the edge of the sun's disc and fades 

 away gradually till at a distance of 

 some 80 to 100 minutes its brilliancy 

 is about the same as that of the sky- 

 background. If a photograph is taken 

 with a very short exposure, only the 

 brighter parts of the corona are regis- 

 tered on the plate. The fainter por- 

 tions do not appear at all. If a pho- 

 tograph is taken with an exposure suffi- 

 ciently long to record the fainter por- 

 tions, all the inner regions of the co- 

 rona are much overexposed, and all de- 

 tail is lost near the sun*s edge. By the 



ordinary methods, then, the corona, as 

 a whole, cannot be exhibited on any 

 single plate. Each exposure is suitable 

 for registering one region, and only one. 

 The corona must be studied on a series 

 of negatives of varying exposures. 



Mr. Burckhalter has devised and tried 

 at two eclipses (the India eclipse of 1898 

 and the Georgia eclipse of 1900) a simple 

 plan which has worked very well. He 

 uses an ordinary photographic telescope 

 and plate, but in front of the plate he 

 places a rapidly revolving shield or dia- 

 phragm, cut to such a shape that dif- 

 ferent portions of the corona have dif- 

 ferent exposures. At the Georgia 

 eclipse, for example, one of his negatives 

 was exposed for eight seconds, but it 

 was, at the same time, screened from the 

 light so that the equivalent exposure 

 at the sun's edge was only 4-100 of a 

 second; at 4' from the sun's edge, Os.32; 

 at 8', 0s.80; at 12', ls.38; at 16', ls.76; 

 at 24', 2s.40; at 34', 3s.20; at 44', 4s.00; 

 at 64',5s.60; at 94' and at all greater dis- 

 tances, 8s.00. The resulting negative is ex- 

 tremely fine, and it exhibits the corona 

 as it has never before been seen on a 

 single plate. The bright inner corona 

 and prominences are shown in their true 

 form and brilliancy alongside of the faint 

 polar rays and the delicate masses of 

 the outer coronal extensions. Those 

 who are especially interested should 

 consult Mr. Burckhalter's report (illus- 

 trated) in the Publications of. the As- 

 tronomical Society of the Pacific, No. 

 75, for October, 1900. The advance 

 over previous work of the same kind 

 is so marked that it is to be hoped that 

 this method will be adopted at the 

 Sumatra eclipse of May, 1901. 



PSYCHOLOGY AS LITERATURE 

 AND FICTION. 



Messrs. Harper & Bros, are re- 

 sponsible for the publication of 'Hyp- 



