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In several respects, they are interesting and remarkable bodies. As to size, they may be said to stand midway between meteorites and satellites. From careful photometric measures executed at Harvard in 1877 and 1879, Professor Pickering concluded their diameters to be respectively six and seven miles.
The chief loss of light in such a burner depends upon the fact that, being circular, the light from the inner surface has to pass through the wall of flame, and careful photometric experiments show that the solid particles present in the flame so reduce its transparency that a loss amounting to about 25 per cent. of light takes place during its transmission.
The results will be published in the form of a catalogue resembling the Photometric Catalogue given in volume xiv. of the Annals of Harvard College Observatory.
No entirely trustworthy means of determining them have, however, yet been found. There is no certainty as to the relative times of exposure needed to get images of stars representative of successive photometric ranks.
I admit that these photometric processes are not very precise; but I believe the last, which perhaps had never before been employed, might he rendered nearly exact, by adding a scale of equal parts to the moveable frame of the telescope of the sextant.
There are drawing, photographic, and photometric chambers, physical, chemical, and metallurgical laboratories. There is a fine lecture-hall, and a splendid library and reading-room. He employs several hundred workmen and assistants, all chosen for their intelligence and skill. In this retreat Edison is surrounded with everything that his heart desires.
Professor Pickering sketched, in 1881, a plan for fixing large refractors in a permanently horizontal position, and reflecting into them, by means of a shifting mirror, the objects desired to be observed. The observations for his photometric catalogues are, in fact, made with a "broken transit," in which the line of sight remains permanently horizontal, whatever the altitude of the star examined.
With a meridian photometer of augmented power, the surprising number of 473,216 settings were made during the years 1891-98, nearly all by the indefatigable director himself, and they afforded materials for a "Photometric Durchmusterung," published in 1901, including all stars to 7·5 magnitude north of declination -40°. A photometric zone, 20° wide, has for some time been in course of observation at Potsdam by MM. Müller and Kempf.
It was succeeded in 1885 by Professor Pritchard's "Uranometria Nova Oxoniensis," including photometric determinations of the magnitude of all naked-eye stars, from the pole to ten degrees south of the equator to the number of 2,784. The instrument employed was the "wedge photometer," which measures brightness by resistance to extinction.
More definite conclusions were, in 1874, derived by Zöllner from photometric observations of Mercurian phases. A similar study of the waxing and waning moon had afforded him the curious discovery that light-changes dependent upon phase vary with the nature of the reflecting surface, following a totally different law on a smooth homogeneous globe and on a rugged and mountainous one.
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