MOON, PHOTOGRAPHING THE (Fr. Photographic de la lune ; Ger., Mond photographie) The photographer who wishes to apply his art to astronomical matters cannot do better than begin with the portrayal of the features of the moon, which, as is well known, is our nearest celestial neighbour. Creditable work may be done with the ordinary hand or stand camera, but it will be understood that with lenses of short focal length it is im possible to obtain images of the exquisite detail which is known to be present on the moon's surface. To obtain such images, a large equiva lent focal length is indispensable, but it is im material whether this is attained by a long-focus objective or by a telephoto combination. Excel lent images showing all the main features of the craters, etc., may be taken with an equivalent focal length of about io ft., giving an image of the moon about r in. in diameter. Many of the standard photographs obtained for charting pur poses, however, have been taken with equivalent focal lengths of as much as 30o ft. For minute study, enlargements can, of course, be employed, but they have the disadvantage of showing up the grain of the original negative, so that if a large scale picture is desired, it is best to use an amplifying lens in the camera. The first success ful photographs of the moon were made by Rutherfurd. Later pictures have been produced by Common, Barnard at the Lick Observatory, Ritchey at the Yerkes Observatory, and Loewy and Puiseux at Paris.
when working with instruments of very large aperture, rapid plates should be employed, so that the minimum exposure may be given, thereby minimising the risk of failure on account of movement of the image. Backed plates are essential. The range of contrast between the bright limb and the craters near the moon's terminator is considerable, and development should be so arranged that the gradation is kept without having to force out detail. As the exposures will, in general, be of minimum dura tion, a soft developer will be most likely to give greatest satisfaction.
Quite recently, d new field of research on the moon has been opened by the discovery by R. W. Wood that certain regions of the lunar surface exercise considerable selective absorp tion. This is especially noticeable when the photographs are taken through screens which only pass ultra-violet light. If a quartz lens is coated with a very thin film of pure silver, it will allow the ultra-violet components of light to pass uninterrupted, but all the visible light rays will be absorbed. Under such circumstances he has found that certain portions of the moon's surface appear darker than when photographed in ordinary white light, and thus it may be inferred that such parts of the moon are composed of some substance which absorbs ultra-violet radia tions.