NATURE 



401 



THURSDAY, AUGUST 15, 1878 



THE COMING ECLIPSE 



"\yy HEN I wrote two articles in Nature a little while 

 ago, discussing the various methods which I 

 thought might with advantage be employed next 

 Monday, I little thought that it would fall to my lot 

 to come to America to take part in the observations. 

 The fates, however, have so ruled it, and here I am, in 

 what was not long ago called the " Great American 

 Desert," but by no means a martyr to science ; for, 

 although Rawlins — where I now am — is nearly 7,000 feet 

 high, and near the Rocky Mountain divide ; although 

 elk and antelope may be shot within a mile of the town ; 

 yet the sluggard is roused at six by the voluminous steam 

 whistle of the railway works : there is a thriving " city" 

 and population. 



The energy displayed by the American astronomers is, 

 if possible, greater than I anticipated. There is scarcely 

 a man of note among them who is not now along the 

 totality line which runs from the Yellowstone Park to 

 the Gulf of Mexico. Where the wonderful Union 

 Pacific Railway cuts the line east and west there 

 will be four stations— Rawlins, Separation, Fulmore, and 

 Creton. Along this line will be gathered Professors 

 Newcomb, Harkness, Draper, Watson, with their many 

 assistants. In the middle region, including Denver 

 Central City, and Pike's Peak, will be Professors Young, 

 Holden, Langley, Cleveland Abbe, and General Myer, 

 the chief of the Weather Signal Service. The parties 

 under these are many of them numerous. Prof. Young' s 

 camp, for instance, including thirteen persons. In the 

 southern region, at Pueblo and Los Animas, Prof. Hall 

 heads a large party, including Prof. Wright, of Yale, and 

 Prof. Thorpe and Dr. Schuster from the old country. 



In all three groups of stations the various kinds of 

 work have been divided in a most judicious manner. In 

 all attempts will be made to obtain the spectrum of the 

 chromosphere and coronal atmosphere in the way sug- 

 gested in my previous articles ; in all the structure of the 

 coronal atmosphere will be carefully inquired into. So 

 far as photographs of the corona are concerned, perhaps 

 the strongest attack will be made by an impromptu party 

 not referred to in the preceding enumeration. On my way 

 here from Cheyenne it was my great good fortune to travel 

 with Prof. Hayden, facile princeps among the great geo- 

 logical surveyors of this vast continent. He was on his 

 way to the north, and, as usual, had with him a strong 

 photographic equipment. As his march lies along the 

 line of totality, he will obtain, or at all events endeavour 

 to obtain, a large series of photographs. 



It is agreed on all hands that never has such summer 

 weather been known in this locality. Ordinarily the 

 chances, as determined by the officers of the signal ser- 

 vice from their registers, are— Northern stations, 80 per 

 cent. ; Denver, 60 ; Pike's Peak, 40 ; and Los Animas, 80 ; 

 but here, for the last fortnight, fine mornings have been 

 succeeded by a break-up in the weather in the afternoon, 

 while at Denver matters have been much worse. 



A most valuable second series of instructions, written 

 by Prof. William Harkness, of the United States Navy, 

 Vol. XVIII. — No. 459 



by direction of Admiral Rodgers, has been pub- 

 lished. Of these Sections I., II., V., and VIII. describe 

 such observations as can be made with ordinary appa- 

 ratus, while the other sections relate mostly to observations 

 which can only be carried out by persons who are able to 

 command expensive apparatus, and who are skilled in 

 astronomy and physics. 



This is a most useful following up of the work of orga- 

 nisation undertaken in England for the first time in 1870 

 and carried out in 1871 and 1875. 



Prof. Harkness has freely availed himself of the Instruc- 

 tions compiled for the English Expeditions of those 

 years, and in his carefully written memorandum has given 

 us an opportunity of seeing how the problems have been 

 advanced of late years ; he has also collected a valuable 

 series of data which give permanent value to it. I do 

 not think I can do better than refer to some of the more 

 important points touched on in the Instructions. 



All the most rapid varieties of lenses in the market suited 

 for use as equatorial cameras are given in the following 

 table, in which the corresponding intensity ratios have 

 been taken from Dallmeyer's catalogue. 



Prof. Harkness points out that " the data from which to 

 determine an approximate value of C for the corona are 

 very limited." He considers that it is probably safe to 

 conclude that, with a clear sky and a moderately high 

 sun, exposures in which the value of C is about o'oo2 will 

 give only the prominences and the outline of the moon* 

 When C becomes o*o8 the corona will begin to appear, 

 and will increase in extent as the exposure increases, at 

 least up to the point where C becomes 0*40. Accordingly, 

 the shortest exposure specified in the table above corre- 

 sponds to C = o'o8, and the longest to C = o"4o. 



If we adopt a lens of thirty-three inches focus an 

 attempt can be made to use the lens for another purpose, 

 " even more important than photographing the corona," 

 that is in the search for intra-Mercurial bodies. Prof. 

 Harkness points out that the magnitude of its intensity 

 ratio enables it to depict faint objects rapidly, and the 

 extent of its angle of view is such as to embrace a field of 

 more than forty degrees. The lens will cover a plate 

 measuring twenty by twenty-two inches, but as it is desir- 

 able to keep the apparatus light, plates measuring seven- 

 teen by twenty inches, which will suffice to cover a space 

 of thirty-three and a half degrees along the ecliptic, are 

 recommended. 



' If i^ is the equivalent focal distance of a photographic objective, d it 

 working aperture, C the exposure constant, whose value depends upon th -. 

 intensity of the light and the sensitiveness of the chemicals employed, and » 

 the time of exposure required to produce a. good negative, then the intensity 



and i 



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