The day of the eclipse was all that could be desired. The sky was almost without a cloud throughout, and the dew point was found to be at least 34° F. below the temperature of the air. The entire programme of observations was carried out as it had been arranged, and with singularly good fortune. "The results obtained," as summarized by Dr. Draper, "were: 1st, 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; 2d, a fine photograph of the corona was obtained, extending in some parts to a height of more than twenty minutes of arc, that is, of more than 500,000 miles; 3d, the Fraunhofer dark lines were observed by both Professors Barker and Morton in the corona; 4th, the polarization was shown by Professor Morton to be such as would answer to reflected solar light; and 5th, 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." As these results seem to be of very considerable importance, it appears desirable to give t^ie various methods of observation somewhat more in detail, adopting for the purpose so far as possible the language of the observers themselves, as given in their several reports.

Photographic And Phototelespectroscopic Observations.

Fig. 1.

The instruments which were used by Dr. Draper in his photographic and phototelespectroscopic observations were: "1st. An equatorial mounting, with spring governor driving clock, loaned by Professor Pickering, Director of Harvard Observatory. 2d. A telescope of five and a quarter inches aperture and seventy-eight inches focal length. furnished with a lens specially corrected for photography, by Alvan Clark & Sons. 3d. A quadruple achromatic objective of six inches aperture and twenty-one inches focal length, loaned by Messrs. E. & H. T. Anthony, of New York; to this lens was attached a Rutherford diffraction grating nearly twoinches square, ruled on speculum metal. This arrangement (Fig. 1.) with its plate holders, etc., will be designated as a phototelespectroscope. Besides these there was a grating spectroscope, an eye slitless prism _*

PROC. AMER. PHILOS. SOC. XVlll. 102. N. PRINTED JAN. 25, 1879.

spectroscope, with two 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 farther description, for success in the work it was intended 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 attain able focal length, and to have a grating of the largest size adjusted in such a way as to utilize 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 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 six inches aperture a,nd twenty-one inches focal length, gave an image of the sun less than one-quarter 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 Rutherford grating set at an angle of sixty 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-shaped 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 totalfty was over and on developing 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, hecause during the totality I had the opportunity of studying the corona through a telescope arranged substantially in the same way as the phototelespectroscope and saw no sign of a ring.

"The plain photograph of the corona taken with my large equatorial 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 elliptical form round 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."

Tasimethic Observations.

As this eclipse is the first in which any attempt has been made to measure the heat of the solar corona, Dr. Edison's report to Dr. Draper on this subject is here quoted in full. He says:

"The instrument which I used at Rawlins, Wyoming, during the solar eclipse of July 29th, 1878, for the purpose of measuring the heat of the sun's corona, was devised by me a short time only before that event, and the time was insufficient to give it as thorough a test as was desirable to ascertain its full capabilities and characteristics.

"This instrument I have named the tasimeter, from the Greek words, Tiktcs, extension, and ftrrpov, measure, because primarily the effect is to measure extension of any kind. The form of instrument which I used is shown in the annexed wood cut (Fig. 2.)

Fiff.2.

"With this instrument was used a Thomson's reflecting galvanometer on a tripod, having a resistance of three fourths of an ohm. The galvanometer was placed in the bridge wire of a Wheatstone balance, two of the branches of which had constant resistances of ten ohms each, while of the other two one had a constant of three ohms, and the other contained the tasimeter which was adjusted by means of the screw to three ohms. When thus balanced, if the strip of vulcanized rubber A (seen in Fig. 3), placed between the fixed point B and the carbon button C, was exposed to heat from any source, it expanded, placing pressure upon the carbon button, decreasing in this way its resistance and destroying the balance ; thus allowing a current to pass through the bridge wire containing the galvanometer, the amount of this current of course being proportional to the expansion of the rubber and to the strength of the battery.

"The form of instrument here described was finished only two days before leaving for the west; hence, I was unable to test it. However, I set it up upon my arrival at Rawlins, but found that it was a very difficult matter to balance so delicate an instrument as a reflecting galvanometer with one cell of battery, through such small resistances. In fact, I did not succeed in balancing it at all in the usual way. Nor could it be balanced in any way until I devised a method which I may designate 'fractional balancing,' when it became very easy to accomplish the result and also to increase the effect by using two cells in place of a single one. This device

Fig. 8.

consisted of a rheostat formed of two rows of pins. The rows were about one-half an inch apart. A wire was connected from a pin on one row to a pin on the other row and so on, so that the current had to pass through the whole length of the wire, which was No. 24 gauge and four feet long. This was used as a shunt around the galvanometer. A copper wire connecting all the pins of one row served to reduce the resistance to zero. When the galvanometer was thus shunted, a very feeble current passed through it. If the spot of light was not at zero it was brought there by either increasing or decreasing the pressure upon the vulcanite of the tasimeter by the adjusting nut. When thus brought to zero the copper wire of the shunt rheostat was taken off of one pin, thus increasing the resistance of the shunt perhaps to one-fiftieth of an ohm. The spot of light was generally deflected nearly off of the scale. The light was again bi ought to zero by varying the resistance of the tasimeter, and another one-half inch of wire included in the shunt, another deflection and another balance was obtained by the tasimeter. Thus by gradually increasing the delicacy of the galvanometer by increasing the resistance of the shunt and balancing at every increase, the whole of the current was allowed to pass through the galvanometer and the shunt taken off. When this point was reached the damping magnet or director was in close proximity to the case of the galvanometer. To increase its delicacy to the fullest extent it became necessary to raise the director to the top of the rod. This was done by raising it cautiously a quarter of an inch at a time, bringing the spot of light to zero each time by the tasimeter.

"In order to form some idea of the delicacy of the apparatus when thus adjusted, a preliminary experiment was made on the evening of the 27th, with the star Arcturus. The tasimeter being attached to the telescope, the image of the star was brought on the vulcanized rubber. The spot of light from the galvanometer moved to the side of heat. After some minor adjustments, five uniform and successive deflections were obtained with the instrument, as the light of the star was allowed to fall on the vulcanite to produce the deflection, or was screened off to allow of a return to zero.

'' It was in this condition when the eclipse occurred. The tasimeter was placed in a double tin case, with water at the temperature of the air between the walls. This case was secured to a Dollond telescope of four inches aperture. No eye piece was used. At the moment of totality the spot of light was slowly passing towards cold. When I withdrew a tin screen and allowed the edge of the luminous corona to fall'upon the rubber, the spot of light stopped, went gradually off of the scale towards heat, its velocity accelerating as it approached the end. The time required for the light to leave the scale was from four to five seconds.

"I interposed the screen and endeavored to bring the light back to zero, but I was unsuccessful. Had I known that the heat was so great I should have used a platinum strip in place of the vulcanite, and decreased the delicacy of the galvanometer by the approach of the damping magnet.

"I would then doubtless have succeeded in getting two or more readings, and afterwards by comparison with bodies of known temperature would have obtained a near approach to the temperature of the sun's corona."

Tblespectroscopic Observations.

My own results, obtained with an analyzing spectroscope attached to the telescope, seem to be almost unique in this eclipse. This fact must be my apology, if any be needed, for introducing here at such length, the facts of the case as contained in my report.

The instruments and apparatus used in the observations were loaned for the purpose from the physical cabinet of the University of Pennsylvania. They consisted (1) of an equatorially mounted achromatic telescope of four inches aperture made by Jones of London ; (2) a direct vision astronom