HARVARD OBSERVING STATION IN PERU.
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.