August 29, 1878] 



NATURE 



463 



and good fortune attended us in every particular. The 

 results obtained were : (i) the spectrum of the corona 

 was photographed and shown to be of the same character 

 as that of the sun, and not due to a special incandescent 

 gas ; (2) a fine photograph of the corona was obtained, 

 extending in some parts to a height of more than twenty 

 minutes of arc, that is, more than 500,000 miles ; (3) the 

 Frauenhofer dark lines were observed by both Professors 

 Barker and Morton in the corona ; (4) the polarisation 

 was shown by Prof. Morton to be such as would answer 

 to reflected solar light ; (5) Mr. Edison found that the 

 heat of the corona was sufficient to send the index beam 

 of light entirely off the scale of the galvanometer. Some 

 negative results were also reached, the principal one being 

 that the 1474 K, or so-called corona, line was either very 

 faint or else not present at all in the upper part of the 

 corona, because it could not be observed with a slitless 

 spectroscope, [a.n6. the slit spectroscope only showed it 

 close to the sun. 



The general conclusion that follows from these results 

 is that on this occasion we have ascertained the true 

 nature of the corona, viz., it shines by light reflected from 

 the sun by a cloud of meteors surrounding that luminary, 

 and that on former occasions it has been infiltrated with 

 materials thrown up from the chromosphere, notably with 

 the 1474 matter and hydrogen. As the chromosphere is 

 now quiescent this infiltration has taken place to a 

 scarcely perceptible degree recently. This explanation 

 of the nature of the corona reconciles itself so well with 

 many facts that have been difficult to explain, such as the 

 low pressure at the surface of the sun, that it gains 

 thereby additional strength. 



The station occupied by my temporary observatory was 

 Rawlins (lat. 41° 48' 50", long. 2h. om. 44s. W. of 

 Washington, height 6,732 feet above the sea), on the line 

 of the Union Pacific Railroad ; because, while it was 

 near the central line of totality, it had also the advan- 

 tages of being supplied with water from the granite of 

 Cherokee mountain, and of having a repair shop, where 

 mechanical work could be done. I knew by former ex- 

 perience that the air there was dry and apt to be cloud- 

 less ; in this particular our anticipations were more than 

 fulfilled by the event, for the day of totality was almost 

 without a cloud and the dew-point was more than 34° F. 

 below the temperature. 



The instruments we took with us were as follows, and 

 weighed altogether almost a ton: — ist. An equatorial 

 mounting, with spring governor driving clock, lent by 

 Prof. Pickering, Director of Harvard Observatory. 2nd. 

 A telescope of S\ inches aperture and 78 inches focal 

 length, furnished with a lens specially corrected for 

 photography, by Alvan Clark and Sons. 3rd. A quad- 

 ruple achromatic objective of 6 inches aperture and 

 21 inches focal length, lent by Messrs. E. and H. T. 

 Anthony, of New York. To this lens was attached a 

 Rutherfurd diffraction grating nearly 2 inches square, 

 ruled on speculum metal. The arrangement with its 

 plate-holders, &c., will be designated as a phototelespec- 

 troscope. 4th. A 4-inch achromatic telescope with Merz 

 direct vision spectroscope, brought by Prof Barker from 

 the collection of the University of Pennsylvania. 5th. 

 A 4-inch achromatic telescope, also brought by Prof. 

 Barker ; to it was attached Edison's tasimeter. Besides 



these there were polariscopes, a gi-ating spectroscope, an 

 eye slitless spectroscope with 2-inch telescope, and finally, 

 a full set of chemicals for Anthony's lightning collodion 

 process, which, in my experience, is fully three times 

 quicker than any other process. 



The arrangement of the phototelespectroscope requires 

 further description, for success in the work it was in- 

 tended to do, viz., photographing the diffraction spectrum 

 of the corona, was difficult, and, in the opinion of many 

 of my friends, impossible. In order to have every chance 

 of success it is necessary to procure a lens of large 

 aperture and the shortest attainable focal length, and to 

 have a grating of the largest size adjusted in such a way 

 as to utilise the beam of light to the best advantage. 

 Moreover, the apparatus must be mounted equatorially 

 and driven by clockwork, so that the exposure may last 

 for the whole time of totality, and the photographic work 

 must be done by the most sensitive wet process. After 

 some experiments during the summer of 1877 and the 

 spring of 1878 the following form was adopted : — 



The lens being'of 6 inches aperture and 21 inches focal 

 length, gave an image of the sun less than \ of an inch in. 

 diameter and of extreme brilliancy. Before the beam of 

 light from the lens reached a focus it was intercepted by 

 the Rutherfurd grating set at an angle of 60 degrees.. 

 This threw the beam on one side and produced there 

 three images — a. central one of the sun, and on either 

 side of it a spectrum ; these were received on three 

 separate sensitive plates. One of these spectra was 

 dispersed twice as much as the other, that is, gave a 

 photograph twice as long. This last photograph was 

 actually about two inches long in the actinic region. If 

 now the light of the corona was from incandescent gas 

 giving bright lines which lay in the actinic region of the 

 spectrum, I should have procured ring-formed images, 

 one ring for each bright line. On the other hand if the 

 light of the corona arose from incandescent solid or liquid 

 bodies, or was reflected light from the sun, I was certain 

 to obtain a long band in my photograph answering to the 

 actinic region of the spectrum. If the light was partly 

 from gas and partly from reflected sunlight, a result 

 partly of rings and partly a band would have appeared. 



Immediately after the totality was over and on de- 

 veloping the photographs, I found that the spectrum 

 photographs were continuous bands without the least 

 trace of a ring. I was not surprised at this result because 

 during the totality I. had the opportunity of studying the 

 corona through a telescope arranged in substantially the 

 same way as the phototelespectroscope, and saw no sign 

 of a ring. 



The plain photograph of the corona taken with my 

 large equatorial (exposure 150 seconds) on this occasion 

 shows that the corona is not arranged centrally with 

 regard to the sun. The great mass of the matter lies in 

 the plane of the ecliptic but not equally distributed. To 

 the eye it extended about a degree and-a-half from the 

 sun toward the west, while it was scarcely a degree in 

 length toward the east. The mass of meteors, if such 

 be the construction of the corona, is therefore probably 

 arranged in an elliptical form round the sun. 



For the fortunate results of this expedition we are not 

 a little indebted to the railroad and express companies. 

 The Pennsylvania, the Chicago and North-western, and the 



