Some of the most beautiful photographs of Mars have been taken by Professor Lowell with his telescope of 24 in. aperture, at Arizona, on an elevated plateau well up out of the lower strata of our atmosphere. He has also succeeded in photographing several of the peculiar thread like markings called " canals," which have caused so much controversy among astronomers during the last twenty years.
Excellent photographs have also been taken by Barnard at the Yerkes Observatory, and by Hale at the Mount Wilson Observatory. The general equipment is the same, and consists of the largest and most perfect telescope that the observer can command ; to this is applied the highest power magnifier that the objective and atmospheric conditions will stand, and also photographic plates of special rapidity and colour sensitiveness. The atmospheric condi tions vary so rapidly that for such objects as planets it is usual to arrange the plate holder as a repeating back, fitted to take a great number of pictures at short intervals on the same plate, so that the few times of best definition may perhaps be caught among a great number of mediocre ones.
Jupiter, although not productive of such great discussion, offers many features for which the application of photography is eminently suitable. This planet is apparently clothed in clouds, and these are arranged in broad belts parallel to the planet's equator, showing as bands of alternately light and dark material on the photographs. In one of them there is a remarkable feature called the " Great Red Spot," a large oval patch of ruddy colour which has been visible now for many years. It is hoped that large scale pictures of this pheno menon may definitely decide the relation of its drift among the surrounding cloud belts, and later, possibly, its true character.
Saturn is the furthermost of the planets of the solar system of which satisfactory photographs have been obtained. Numerous excellent pictures have recently been taken at the Lowell Observatory, showing most minute detail on the cloud belts and the surface of the rings.
Uranus and Neptune, the two outermost planets, have been often photographed, but their images so far have only been small discs without definite surface detail.
The minor planets are a closely associated swarm of small bodies revolving round the sun in orbits between Mars and Jupiter. The majority of them are so small as usually to be unobservable even with the largest telescopes, and they have nearly all been discovered by means of their trails on photographic plates exposed for long periods to the sky. The early part of this work was done by moving the telescopic camera at the same rate as the earth's rotation, so that each photograph showed all the stars as minute bright points. If then an object was found showing an elongated image, and it was decided to be a real marking, it was assigned a provisional number as a new planet, and after full confirmation on subsequent photographs d. name would be given to it. Lately an ingenious method has been devised to permit of the discovery of small planets being recorded which, if left to trail, would be too faint to give any perceptible image. Suffi cient is now known of the general motions of this belt of small planets to allow of their probable velocity in any region of their orbit being fairly accurately calculated. The camera is then set
to travel at this velocity, instead of at the earth's velocity as before. This means that the little planet will now be able to accumulate its light on the image on the plate, and it will thus show up as a small round point among the surrounding stars represented by short elongated trails. The plates used should be of the fastest emulsion available, consistent with a fine grain.
Nebula Photography.—This branch of photo graphic astronomy is almost beyond the reach of the camera alone. There are only one or two nebulae which can be photographed with rapid exposures, and then only on a very small scale, scarcely repaying the trouble which may be taken. With a small expenditure, however, a very serviceable driving mechanism may be obtained or constructed by the observer, and then very interesting pictures of the chief nebulae may be photographed without any greater difficulty than is experienced in taking high power telephotographs. The focal length of lens to be selected depends to some extent on the object selected for experiment. Thus the ne bulae of Orion, Andromeda, the Magellanic Cloud, the Pleiades, and other objects of considerable extent give very beautiful pictures with cameras of quite moderate focal lengths, say from 12 to 3o in. Naturally on such objects the lens should be of the largest relative aperture possible, as then the exposure necessary will be the minimum, and the chances of disturbance during the operation reduced as far as possible. The range of gradation is so great that the utmost care should be taken with the development, a moderately dilute solution of any of the standard formulae being suitable. All plates should be thickly backed, otherwise the star images distributed over the region will be so expanded as to detract seriously from the value of the picture.
The famous Orion, or " Fish Mouth," nebula was first satisfactorily portrayed by photo graphy by Draper, in 1880. Later magnificent pictures were obtained by Dr. Common in 1883, at Ealing, with a large silvered glass reflector, 36. in. in diameter, which he had himself entirely constructed. With further advances in the rapidity of photographic plates and perfection of apparatus it was possible to delineate greater and fainter extensions, and also by employing instruments of enormous length the fine detailed structure was for the first time brought to our knowledge. The greatest advances of recent years were made by Keeler, at the Lick Observatory, California, and Ritchey, at the Yerkes Observatory, near Chicago. The former made an extensive survey of the class of nebulae whose structure is dis tinctly spiral, and published an atlas of beautiful reproductions of the most important. Ritchey has more specially aimed at obtaining specially large-scale pictures of the principal nebulae, showing the wonderful relationships existing between the cloud-like wisps and the star aggregations with which they are apparently associated. The study of these pictures is of the greatest importance.