Agreeably to the announcement of the annual report of Harvard College Observatory for 1877, as to photometry, a beginning was made by constructing a photometer suitable to be attached to the great telescope. Other photometers have been devised at different times for use independently. One of the earliest was applied during the year beginning Oct. 12, 1877, in measuring the light of all known satellites excepting the two inner ones of Uranus, which are too faint to be discerned, even by the great telescope. The first prolonged observation entered upon was of the eclipses of Jupiter’s satellites.

As there are four satellites and as the plane of their orbits is nearly the same as that of the planet itself, eclipses are frequent. The plan proposed the observation of all these eclipses visible during a revolution of Jupiter around the sun, a period of about 12 years. The work was begun June 23, 1878, and has been regularly pursued. The final result will be of the highest value in that, among its utilities it will permit a new and independent computation to be made of the earth’s distance from the sun, which distance is a prime factor in theoretical astronomy.

Computations hitherto made, based upon data derived from these eclipses, are not authoritative, because of disagreements among different observers using different telescopes, and because of defects in the method of observation.

The director’s report for 1878 says: “Errors of this kind are much lessened by photometric observations of the satellites as they gradually enter or emerge from the shadow of Jupiter, using the planet itself or another satellite as a standard. Each comparison thus obtained gives an independent determination of the time of the eclipse, free from the errors due to the condition of the air or the power of the telescope employed and less likely to be affected by personal equation than the observation of a disappearance or a reappearance. By the ordinary method an observation during twilight can have little value, while good photometric observations may be made as well then as at any other time. It is even possible to make them before sunset.”

In 1879 a work of magnitude was begun—the photometric observation of all stars down to those of the sixth magnitude visible in this latitude. For greater facility, and particularly to avoid loss of time in identifying stars of small magnitude, it was decided to make a new departure in method and in construction of an instrument. The new instrument was called the meridian photometer, and stars were observed by it only when near the meridian. The position of any star being well known, the time of its appearance in the field of the telescope could be foreseen.

Each that was desired for a particular night had, therefore, only to be waited for, not sought for. The original instrument consisted of a fixed horizontal telescope pointed west and having two objectives.

The light of the pole star, which was taken for the standard or unit of measurement, was reflected by a prism into one object glass, and that of the star to be measured into the other. The light of the brighter star was then reduced to exactly that of the fainter by the turning of a screw having a register attached. The indication of the register gave the measure, which was confirmed by repeated observations. Telescopes mounted in the ordinary way continued to be used in other branches of photometric work.

The photometric survey of the sixth magnitude and brighter stars was completed Aug. 25, 1881. In 1882 a new and more powerful meridian instrument was constructed and a photometric survey of a list of about 21,000 stars, from the sixth to the ninth magnitude, was entered upon. This work was finished Sept. 29, 1888, and soon afterwards the instrument, with others, was sent to Peru in charge of Mr. S. I. Bailey of the observatory corps, where, May 11, 1889, a corresponding survey of the stars, from the first to the ninth magnitude, inclusive, between 30° south and the southern pole, was begun. Thus the facts relating to all the stars in the sky of these classifications will be embodied in the final result.

The record, which will comprise several volumes, one or more of which have already been published, will have an identity throughout as respects the method, the instrument, and the unit of measurement. It will be authoritative as a text book or series of text books, and will enhance the value for reference, and comparison of various records of the light of stars, both those of modern and ancient date.

The successful working of the two meridian photometers led to the construction of one still more powerful, having an aperture of 12 inches. The first was of 1½ inch aperture, and the second of four inches.

The three differ somewhat in mechanism, but are the same in principle. The 12-inch is called by distinction the “horizontal telescope.” It will be available in case a photometric survey of stars of fainter magnitudes shall be undertaken, but its use is not limited to photometry.

In 1879, a photometer was devised for measuring the light of nebulÆ, thus applying to these objects and to stars the same unit and scale. In 1881, photometric observations of certain bright parts of the moon, were made for the Selenographical Society of England, the particular parts being selected by that society. It thus was shown that the lunar scale of light in common use may be closely expressed in terms of stellar magnitude, each degree of the lunar scale answering to six-tenths of a magnitude. Photometry has been very extensively applied at Harvard in study of variable stars.

A history of any department of practical astronomy, written from the point of view of a mechanician, could hardly fail to be of interest. Among the curious experiences at Harvard in the line of photometry is one which illustrates this point, and, at the same time, indicates the refinements in observation which are resorted to, and demonstrates one of the utilities of the photometric method.

In 1877 announcement was made of the discovery at Washington of two satellites of the planet Mars. The Harvard telescope being applied they were after a little effort descried as two faint points of light, showing no visible disks. To ascertain the diameter of each satellite might therefore seem impossible: but it was done, approximately, by the photometric method.

The mechanical problem was to reduce the light of the planet as seen in the telescope to an equality with the light of one of the satellites as thus seen. Five or six different mechanics were employed to drill in a piece of metal a hole, making a true circle, and small enough to produce the equality sought for by sufficiently diminishing the light of the planet. It may be remarked that one of those who succeeded best had already, for his own purposes, managed to drill a hole, lengthwise, through a fine cambric needle, making a steel tube of it.

What he made for Prof. Pickering was a hole in a steel plate, the diameter of which was one eighteen hundredth (1-1800) of an inch. It was so nearly circular that the various diameters, including errors of measurement, only differed one one hundred thousandth (1-100,000) of an inch.

Other mechanical devices were resorted to for corroboration, and the results reached were that the diameter of one of the satellites is about six miles, and that of the other about seven miles. They are the smallest known in the solar system.

The availability of the spectroscope in astronomy had early been appreciated by the profession. In experiments in this line it had been found that a classification of the nebulÆ might be made upon the basis of their spectra. In 1880 the study was carried a stage further at Harvard in ascertaining by the spectroscope that certain faint objects, which, by direct vision, had been judged to be stars, are in fact nebulÆ. In 1881, it was found that the spectroscope is serviceable in the discovery of variable stars. Thus incited, a new instrument was imported from London, but it did not prove satisfactory.

Nothing of importance appears to have been done in this department thereafter until 1886, when the proposition of Mrs. Draper opened the way to investigation of spectra by aid of photography. For this the 11-inch photographic telescope, which had been used by Dr. Draper, was loaned by Mrs. Draper, who also met the expense of a new mounting and a special observatory building. A beginning was made with an eight-inch instrument, known as the Bache telescope. It is of the pattern described as the “doublet,” and offers the advantage of a large field of view. With it the spectra of about 10,500 stars of the sixth magnitude and brighter, between the pole and 25° south, were photographed before the close of the year 1888.

The instrument was then sent to Peru, where a like survey of the Southern sky is in progress. Spectroscopic observations of the brighter stars have been continued at Cambridge with the 11-inch Draper telescope and of fainter stars with an 8-inch doublet similar to the Bache instrument. In this work it was found that by giving a certain chemical stain to the photographic plate the yellow and green portions of the spectrum of even the fainter stars can profitably be studied.

Furthermore, what seems incredible at first thought, it appears to be demonstrated that the components of binary stars whose juxtaposition does not permit them to be separated in any telescope, may, by spectroscopic photography, be shown to be in revolution about each other. Two or more such objects have been found in which the changes regularly succeeding in the lines of the spectrum not only prove that the components are in motion, but permit the period of revolution to be determined.

Prior to 1883 photography is mentioned in the annual reports of the present director only as incidental to other work. In that year a systematic investigation was undertaken, having among other objects in view, the construction of a photographic map of the whole heavens. An early application of photography in this investigation was in the direction of determining the color of stars, measuring their brightness by an independent method, picturing their spectra, exhibiting the effect of atmospheric absorption of light in a series of plates covering the period of a year, and ascertaining by images of stars trailed upon the plate, the clearness and steadiness of the atmosphere.

In 1887 the Boyden fund being available, the first step was taken in the important enterprise of giving a continental expansion to the work of the observatory. The aim of the testator in making his bequest could well be furthered in conducting observations simultaneously in photometry, spectroscopy and photography. In following up the project, the Draper memorial funds appear also to have been available to a considerable extent in the two latter methods of observation. Experimental stations were established in Colorado in the summer of 1887 on mountain peaks of 14,000, 11,000 and 6000 feet in height, respectively, and the meteorological conditions, including the transparency and steadiness of the upper atmosphere, were duly tested.

This investigation was continued at the expense of the Boyden fund during the following winter by local observers whose stations were at considerable height.

In 1889 the movement was further extended by establishing an observatory on a peak about 6500 feet high in Peru, 25 or 30 miles distant from the sea coast and the city of Lima. Local official sanction was given to naming the peak, “Monte Harvard.” About the same time other observers of the Harvard corps set up an experimental observatory on Mt. Wilson, 6000 feet high, in Southern California. The station is about 30 miles from the sea coast and somewhat less from the city of Los Angeles.

The experimental purpose is the same as in Colorado, and looks to the ultimate establishment of a permanent observatory as a branch of the Harvard institution at some favorable point where the superior atmospheric conditions of the Pacific mountain regions can be had. In the special direction of picturing celestial objects at Mt. Wilson remarkable photographic results are already possessed at Cambridge in plates showing lunar surfaces, Saturn’s rings, Jupiter’s belts and the most brilliant of the nebulÆ. That among them which is of the greatest scientific interest, as being a novelty, is the picture on a negative plate of the great spiral nebula of Orion. It is a Harvard discovery by the photographic method, and is quite other than that heretofore known as the great nebula in Orion. That is an object having a span of about half a degree. The new great nebula has a span of nearly 17 degrees; its outline includes all the stars of the constellation, and it is too faint an object to be discerned by the naked eye.

It is one of the principal advantages of the photographic method in astronomical work that the sensitive plate will denote objects which the eye reinforced by a telescope of any power cannot detect. The great nebula thus discovered is within reach of the telescope, but its dimensions are so much larger than the field of the telescope, and its outline so faint, that its true character would not thus originally be apprehended.

Photography at Cambridge has already produced several series of plates, each plate covering a section of the northern sky, the whole of which when perfected and collated will be a self-recorded, and so, indisputable atlas, showing the position of all stars down to those of the 11th magnitude. It will be an atlas in sheets of glass, and frailer in some respects than if composed of sheets of paper. But for study of the science the glass is better than any product of the engraver’s art, and better than any sun picture printed by the plate itself. Indeed, it is one of the triumphs of the photographic method that a perfect photographic negative discloses more to the student than does a telescopic view of that area of the sky of which the photograph is a copy. Astronomical research is now constantly made at the observatory in this manner, and with results equal to or better than those reached by former methods.

Celestial objects are thus originally discovered and the positions of familiar objects remeasured or otherwise compared, and this work might be continued throughout the whole 24 hours were it so desired, regardless of the glare of the sun by day or of impenetrable clouds by night.

The work in progress in Peru will give other series of plates offering equal facilities for the study at Cambridge of that part of the sky which is beyond our southern horizon. Some of the results which these extensive investigations of the light, the spectra and the positions of the stars will yield will anticipate the doings of other great observatories of the world. But there is no necessary limit at stars of the magnitudes named; there will remain other worlds to conquer.

A special encouragement to new enterprises at Harvard is in the munificent gift of $50,000, made within the year past by Miss Catherine W. Bruce of New York for the construction of a telescope of 24 inches aperture, to be used in photography. A contract for this instrument has been made. It is intended that its first use shall be to photograph maps of the fainter stars, and it is hoped that those as faint as the 16th magnitude can thus be represented. The basis of this sanguine forecast is the fact that with an eight-inch telescope of the pattern of the proposed 24-inch, and an exposure of the plate for one hour, twice as many stars are photographed as are visible with a telescope of 15 inches aperture. Prof. Pickering received the honorary degree of A.M. from Harvard in 1880, and that of LL.D. from the University of California in 1886, and from the University of Michigan in 1887. Like his predecessor, Prof. G. P. Bond, he has been honored by the Royal Astronomical Society in the bestowal of its gold medal.

The several investigations of chief importance which are now in progress at Harvard College Observatory have already been mentioned as part of the record of the half-century past. They also go into the record with which the second half-century now begins. As such they may be briefly recapitulated, viz.: The survey, for the purposes of the great European standard catalogue known as the “Astronomische Gessellschaft,” of the zone between 9° 50' south and 14° 10' south; the photometric, spectroscopic, and photographic special surveys making in south latitude to complete like surveys hitherto made at Cambridge, extending to about 30° south; the systematic work in photography, which includes much classifiable as spectroscopy, carried on both at Cambridge and in Peru as the Draper Memorial work: other systematic work of like importance done under the special restrictions of the Boyden fund; and what perhaps may be called the orbital observations of eclipses of Jupiter’s satellites.

That planet has now nearly completed its circuit around the sun, and the last of its satellite eclipses to be observed will occur on Dec. 17 ensuing. During the period of 12 years about 450 of these eclipses have been observed and recorded. Perhaps as many others for which preparations were made at the observatory, passed unseen, because of interposing clouds. Except to an expert these figures give no hint of the magnitude of the work. All that need here be said is that in its completed form it will be one of the great achievements of the observatory.

The enumeration of these unfinished works and those completed, which has now been made, will have fulfilled its purpose if it shall have impressed upon the mind of the general reader the fact, with which it is presumable everybody is somewhat familiar, that a great oak has grown from the little acorn planted on Harvard College campus 50 years ago.