ART. XXXII.

Observations upon the Solar Eclipse of July 29, 1878, by the Princeton Eclipse Expedition; by Professor C. A. YOUNG.

THE Princeton expedition to the Rocky Mountains to observe the recent eclipse, was organized early in the year, and its expenses were mainly provided for by a liberal appropriation made for the purpose by the trustees of the estate of the late John C. Green. I have said, mainly, because we are also greatly indebted to the kindness of the managers of the Pennsylvania, the Chicago and Alton, and Kansas Pacific Railroads, and to the American Express Company, for the free transportation of persons and instruments, to the authorities of the State of Colorado for the loan of camp equipage, and to the Western Union Telegraph Company for various courtesies. We are also under obligations to Mr. Edison, to Rutgers and Dartmouth Colleges, and to the observatory of Harvard College for the gift or loan of apparatus.

Our principal object was to investigate the spectrum of the corona and chromosphere- not only the visible portion, but also, and especially, the invisible portions below the red and above the violet. It was hoped that some new lines might be discovered in these portions by the help of the thermopile, photography or fluorescence, but in this respect, as will be seen, our hopes were not fulfilled. Indeed we were well aware from the outset that the chances were considerably against us, and that it was quite likely, as it turned out, that the corona would sympathize with the present general apathy of the solar surface to an extent which would make the bright lines of the corona spectrum unusually faint and difficult of detection.

Our party, under the charge of Professor Brackett and myself, consisted of ten persons when we left Princeton on July 1st. We were joined in Missouri by another, and after we went into camp at Denver by still another, making up our proper party to twelve persons. Mr. Ranyard of the Royal Astronomical Society was also with us as our guest, and observed from our camp, and on the day of the eclipse we were assisted by several volunteers who came out from the city for the occasion.

Arriving in Denver on July 5th, we went into camp on the 7th in a grove on the bank of Cherry Creek about two and a half miles southeast of the city. Our position was determined by a triangulation made by Messrs. Libbey and McNeill, connecting us with several well determined points in the city.

Assuming for the High School house in Denver, lat. 39° 45′

AM. JOUR. SCI.-THIRD SERIES, VOL. XVI, No. 94.-Oct., 1878.

006", long. 1h 51m 45-2m west of Washington, according to the data kindly furnished by the Coast Survey, the position of the pier of our equatorial was, lat. 39° 43′ 27′′, long. 1h 51m 408.4 west of Washington. The other instruments were all within 150 feet of this. Our equipment was as follows: For time, we had a sidereal box chronometer, with electric break circuit, by Parkinson & Frodsham, No. 4121; also a mean time pocket chronometer by the same makers, No. 5450. The rates of both were small and very regular. Their errors were determined every fair day by altitudes of the sun measured with a Pistor & Martin's prismatic sextant and artificial horizon. It was only rarely that we could get equal altitudes in morning and afternoon, as the afternoons were usually cloudy even when the mornings were fine. For chronographs we had two Morse registers of the European pattern.

For the ordinary ocular observations we had: (1) A four-inch telescope by Clark-the object glass of the meridian circle of the Princeton Observatory-temporarily mounted in a rough tube upon a very rude altitude and azimuth stand. It was fitted with a Herschel solar eyepiece and power of about fifty. Professor Brackett used this in observing the first and last contacts. (2) A four-inch dialytic by the late Mr. Sage of Orange, N. J., equatorially mounted. This instrument was kindly loaned by Mrs. Sage at the request of Professor C. G. Rockwood, who observed with it the first and last contacts, assisted by Mr. J. C. Grant. (3) A telescope of two and three-fourths inches aperture, the object glass, by Clark, belonging to the Princeton transit instrument, mounted upon a rough equatorial stand, and provided with a screen for observing the projected image of the sun. (4) A comet-seeker by Fitz, six inches aperture and about forty-six inches focus, with a curious arrangement of two eyepieces: power 19.5. This was mounted upon a tripod with altitude and azimuth motion. It was used by Mr. Malcolm McNeill during totality in sweeping for intra-mercurial planets, but without success.

After totality the large equatorial of the Princeton Observatory, having nine and a half inches aperture and twelve feet focus, was used by myself with the Merz polarizing eyepiece and power of 250, in observing the cusps and the moment of last contact.

For observing the visual spectrum of the corona four instruments were provided. (1.) The large equatorial with clockwork just mentioned. This was fitted with a single-prism spectroscope, and fluorescent eyepiece, having a film of solution of Esculin about 1mm. thick. With this the dark lines of the ordinary solar spectrum could be easily seen as far as O, and with precautions I could see even farther. The eyepiece could be set axially as well as obliquely. I used this instru

ment myself during totality. (2.) The finder of the equatorial is a fine telescope of three inches aperture. To this was fitted a spectroscope with a diffraction grating on silvered glass by Rutherfurd, 17,280 lines to the inch. With this I observed the first contact, and the behavior of the spectrum up to totality. During totality it was not used. I was assisted during the eclipse by my son, Mr. C. I. Young, who pointed the telescope and made my records.

(3.) A single-prism integrating spectroscope mounted equatorially, with an opera glass in front of the slit as a condenser. The collimator and telescope had each an aperture of two and a half inches, and a focal length of twenty-six and a half. The prism, kindly loaned for the occasion by Professor Emerson of Dartmouth College, had a refracting angle of forty-five degrees, and faces two and a quarter by two and a half inches. It was mounted, not at the angle of minimum deviation, but in such a way that by moving a lever it could be slightly rotated so as to throw the spectrum across the field of view. The whole was mounted equatorially, but without clockwork, and was committed to Mr. C. D. Bennett. (4.) An integrating spectro scope of high dispersion by Grubb. This had telescope and collimator of one inch aperture and twelve inches focus, with a train of dense sixty-degree prisms, varying in number at pleasure from ten to four-six were used during totality-it was fitted with an opera-glass condenser like the preceding. The dispersive power was too great, and nothing at all was seen with it by Mr. H. S. Smith who had it in charge.

(5.) For the observation of the spectrum-images of the corona, a slitless spectroscope of peculiar arrangement was employed, constructed specially for the purpose by Clark & Sons. Two small telescopes, precisely similar, each of two inch aperture and fourteen inch focus, with a magnifying power of ten and a half were placed parallel to each other upon a board, and in front of their object glasses were secured two thirty-degree prisms each four and a half inches long by two and a quarter wide. One face of the anterior prism was set so as to be perpendicular to the incident light, and one face of the other prism was perpendicular to the optical axes of the telescopes. The whole affair was mounted equatorially upon a post by Professor Rockwood (who had charge of the instrument) in a very ingenious manner, so that the plane of dispersion could be altered at pleasure. The three inch telescope before mentioned was attached to the same stand and arranged for polarization observations during the totality, but Professor Rockwood did not get time to use it. During the totality, Professor Brackett also examined the coronal images as seen by looking with the naked eye through a direct-vision prism of considerable dispersion.

Our photographic attack upon the corona spectrum employed four instruments. (1.) A spectroscopic camera with diffraction grating of 8640 lines to the inch, the ruled surface two and one-fourth by one and three-fourth inches. The collimator had an achromatic lens of three inches aperture and about forty inches focus; the slit was about three-fourths of an inch long, and so arranged that one-half of it could be uncovered at a time, for the purpose of securing the dark lines of the solar spectrum as reference marks by a short exposure of one-half the slit after totality, the other half having been exposed during the totality. The image was formed upon the plate by a quartz lens two inches diameter and twenty inches focus. The light was concentrated upon the slit by an opera-glass condenser with quartz lenses. The whole was equatorially mounted upon a post. With sunlight this instrument gave strong and beautifully defined impressions of the spectrum from F to O with exposures of between one-fourth and one-half a second; but the plate exposed through the whole of the totality showed no trace of action, to the great disappointment of Mr. W. Libbey, Jr., who had charge of the instrument and undoubtedly got from it all it would do.

(2.) A prismatic camera, consisting of a slit, a prism and a single quartz lens of ten inches focus. This instrument also had an opera-glass condenser, but of glass lenses, and was mounted equatorially upon a post. The rays from the slit were not rendered parallel before passing the prism, the lens being between the prism and sensitive plate. The slit of this instrument was arranged so that exposures of different lengths could be given to different adjacent portions of the plate. It gave impressions even more quickly than the preceding in full sunlight, but like it, failed to give any result during totality, notwithstanding the skillful management of Mr. W. W. McDonald, to whom it was intrusted.

(3) and (4) were slitless spectroscopes mounted together upon an equatorial stand kindly loaned to us by Professor Pickering, of the Harvard College Observatory. Professor Brackett, with great skill and ingenuity, attached to this an excellent clockwork compiled for the occasion mainly from the movement of the chronograph of the Princeton observatory. In one of the instruments the dispersion was effected by a speculum-metal grating of 17280 lines to the inch, with a ruled surface two and one-fourth by one and three-fourth inches. In the other a sixty-degree prisin of white flint was used. image-forming lens in each case was an achromatic of about two and one-half inches aperture and twenty-four inches focus. About eight minutes before totality, the diffraction instrument, with a five-second exposure gave a fine spectrum, but neither

The

of them succeeded during totality. Professor Brackett, assisted by his wife, took this double instrument as his special charge, though he besides had the entire management and oversight of all the photographic work; and not only so, but nearly everything that was ingenious in contrivance and skillful in execution was his also.

For photographing the corona, we had a six-inch telescope by Clark, with an object glass specially corrected for the actinic rays, loaned us by Professor Pickering. This was mounted upon an equatorial stand with clockwork, lent to us by Rutgers College. Mr. G. H. Calley was put in charge of this instrument and obtained with it three fine pictures of the corona with exposures of ten, fifteen and twenty-five seconds respectively. A plate which was to have had forty seconds exposure was unfortunately lost.

Only one instrument remains to be mentioned, that with which we attempted to explore the infra-red portion of the spectrum. It was a spectroscope, with thermoscopic apparatus substituted for the retina. A tube about five feet long carried at the upper end a slit two inches long taken from a Duboscq electric lantern. At the lower end of the tube was mounted a large spectrum-metal diffraction grating 5760 lines to the inch, in such a manner that it could be slightly turned by moving a long lever. In front of the grating was fixed a twoinch quartz lens, at the end of a bifurcated tube, suitably diaphragmed, and carrying at the other end a delicate line thermopile and a tasimeter. The whole apparatus was attached to a board equatorially mounted. In a large box close at hand was placed a delicate differential reflecting galvanometer, constructed by Professor Brackett specially for the purpose―as, indeed, was also the thermopile-both galvanometer and thermopile distinctly superior to anything we could otherwise

obtain.

We

We had relied mainly upon the tasimeter, which Mr. Edison himself constructed, and gave to us, with a wholehearted generosity which deserves and has our sincerest thanks. received the instrument, however, only the day before leaving home, and when we came to experiment in the field we found it quite impossible to do anything with it without completely remodelling the whole apparatus, which there was no time to do. Whenever the direction of the instrument was changed to keep the collimeter pointed to the sun, the whole adjustment of the tasimeter was destroyed. It needs to stand unmoved upon a firm foundation, while the rays under investigation are brought to it by reflection. Under the circumstances we were, therefore, obliged to rely wholly upon the thermopile.

Mr. A. D. Anderson managed the lever which moved the