Towler, John. The Silver Sunbeam. Joseph H. Ladd, New York: 1864. Electronic edition prepared from facsimile edition of Morgan and Morgan, Inc., Hastings-on-Hudson, New York. Second printing, Feb. 1974. ISBN 871000-005-9

Chapter XLII.

WITHIN the last few years the importance of the application of photography to astronomical investigations, as well as to meteorology, has been recognized by natural philosophers as a definite help-mate in the prosecution of these studies. This application consists, when referred to astronomy, in obtaining photographs and stereographs of the moon in its various phases, of the sun, of the planets, and of the comets; and, when referred to meteorology, in obtaining photographic delineations of the different classes of clouds; of the aurora borealis in its various configurations, of meteors, halos, water-spouts, paraselenes, etc.

For this purpose the various forms of telescopes, both reflecting and refracting, may be employed, which are used in observatories. Refracting telescopes, from the fact of their objectives being corrected for the luminous part of the spectrum, are far from being corrected for photographic purposes; and the ground glass has sometimes to be moved as much as an inch from the position of the luminous focus before the actinic focus is arrived at. Reflecting telescopes, on the contrary, not decomposing light, have to undergo no correction for actinism. For amateur astronomical photographers silvered glass mirrors, as recommended by Steinheil, are most easily constructed, are comparatively cheap, and from their lightness very manageable. These, when properly mounted, will give a picture of the moon, etc., of a magnitude varying with the focal length of the mirror. The ground glass is placed in the principal focus of the objective or reflector, and has a motion by which it can be adjusted to accuracy. In such cases the eye-pieces are removed. When the operation is not instantaneous, the telescopes have to be furnished with clock-work, by means of which the axis of the body photographed can always be made to coincide with the axis of the telescope, during the time of exposure.

We are indebted to Messrs. Bond of Cambridge, Crookes, De la Rue, Hartnup, Forrest, Edwards, Berry, Hodgson, Secchi, etc., for interesting information and photographs of celestial objects.

The moon naturally claimed the first attention; the Bonds were the first to obtain a daguerreotype of this satellite; Messrs. Hartnup, Forrest, Berry, and Edwards obtained very beautiful photographs on collodion of the moon in 1854, of an inch and one third in diameter, which they afterward magnified a few diameters on another collodion plate, and then exhibited the photographic representation on a large screen sixty feet in diameter, the picture having been magnified to this extent by means of a magic lantern.

At the same time Mr. Hartnup suggested a plan of taking a stereograph of the moon, by first taking a photograph of this luminary twelve hours before full, and then twelve hours after full, thus changing the shadows from one side. to the other. The stereograph was successful, and exhibited the moon in relief. From the fact that the moon revolves on her axis in the same period that she revolves in her orbit, it is difficult to obtain a degree of parallax by which more of one side can be seen at one time than at another; but the axis of the moon, that is, the moon itself in the direction of its axis, has a sort of libration, which brings to light alternately more of the northern parts than of the southern. By taking photographs at these different periods a sufficient amount of parallactic angle has been obtained, and perfect stereographs have been the result. The moon has been photographed and stereographed in all her phases; the shadows of the mountains are so well delineated in these different phases as to admit an accurate measurement of the height and diameter of those mountains.

A few seconds' exposure with a good equatorial will, in general, give a tolerable negative. It is not absolutely necessary to be furnished with a telescope in order to get a photograph of the moon; the photographer will be glad to learn that a long-focussed view-tube will permit him to obtain a copy of this bright luminary, of about half an inch in diameter, which can afterward be magnified to any size desired. The only difficulty is to keep the moon in exactly the same position of the ground glass for a number of seconds.

The sun is easily photographed, because the operation is instantaneous. The act of focussing is performed by placing a piece of violet-colored glass over the opening of the objective, which is retained there during the exposure. In this way eclipses of the sun can be photographed without that immense trouble and risk which have been so often referred to in celestial photography.

The stars have a high degree of photogenic power, and can be photographed in accordance with the intensity of their light. Mr. Bond has endeavored to base upon this power a means of classifying the stars into magnitudes, which at present are quite arbitrary.

The planets, possessing much less photogenic power than the fixed stars, are in consequence not so easily photographed; but by means of well-regulated clock-work, De la Rue has succeeded in obtaining very excellent photographs of Jupiter and his bands, Saturn and his rings, as also a stereograph of Mars, by taking two photographs at an interval of two hours, and others of Saturn at an interval of three years and a half.

It is supposed there are planets nearer to the sun than Mercury, which have not been discovered by reason of their proximity to the solar orb: it is hoped, however, by means of photography to settle this supposition; for if at any time in any of the numerous photographs which are taken of the sun, a small, round black speck should be discovered, the conclusion of the existence of another inferior planet would soon be drawn.

De la Rue is prosecuting this branch of photography and astronomy with great zeal and success; his labors, too, are highly appreciated. The Astronomical Society of London have conferred upon him their annual large medal as a token of their high appreciation of his merits and the results of his labors.