The largest lens in actual use is that at the Yerkes Observatory, near Chicago, which has an aperture of 4o in. and a focal length of about 6o ft. Numerous reflecting telescopes are in use. The two largest now being used for photo graphy are 6o in. in diameter ; one, made by the late Dr. A. A. Common, is now at Harvard College Observatory, U.S.A. ; the other, made by G. W. Ritchey, is at the Solar Observatory, Pasadena, California. This last is probably the most powerful telescope the world has ever known, and the resulting photographs are of wonderful clearness and beauty. Many re searches are being made with this 6o in. reflector which are quite beyond the scope of any other instrument, such as large scale photographs of the spectra of stars and planets, measurements of the velocities of stars in space, etc.
One of the greatest applications of the equa torially mounted star camera has been the " International Photographic Chart of the Sky," a co-operative scheme shared by all the nations of the world. This was inaugurated at Paris in 1887, when a congress of the world's astro nomers resolved to prepare a photographic chart of the heavens, showing stars down to the fourteenth magnitude. This was to be supple mented by a second series of plates of short exposure, for a catalogue of stars down to the eleventh magnitude. The standard instrument was a twin refracting telescope—a photographic camera of 13 in. aperture and i r ft. focal length, attached to a visual guiding telescope of it in. aperture. Each photographic plate covers about four square degrees of the sky. This magnificent work is now being brought to a successful conclusion, and the publication of the charts is well advanced.
A task of such magnitude is obviously not applicable to the detection of rapid changes in the positions or brightness of the stars. To provide information for this purpose Pickering, at Harvard College Observatory, has arranged for a complete photographic survey of the sky with a short focus camera. Copies of this work are obtainable, and it will be repeated at con venient intervals, so that a definite check will be possible on any changes in the positions or magnitudes of the stars.
For charting very condensed areas, such as the Milky Way, in which it is important to obtain a record of the general aggregation of the stars, the most successful method is that adopted by Barnard, who used at the Lick and Yerkes Observatories a small lantern lens of 2 in. aperture and 5 in. focal length. An
apparatus of this type gives exquisite pictures of the star groupings when used with an efficient equatorial mounting.
Photography is now used for systematically recording the brilliancy of stars. This is done by obtaining photographs with the plate slightly displaced from the true focus for parallel rays, so that the image of each star shows as a small round disc instead of a very tiny point. A series of standard density squares from a standard light source are impressed on the plates before exposure to the sky ; it is therefore possible by measuring these density squares and the star discs to obtain accurate values of the relative brightness of the stars.
Planet Photography.—This is so much beyond the reach of the ordinary photographer that it is only suitable here to give a short outline of the results obtained by the photographic astronomer provided with the very special equipment necessary for the purpose.
The first successful attempts to obtain pictures of the planets showing the details of their surface markings were made by Gould, at Cordoba, in 1879, on the planet Mars, and by Dr. Common in the same year on Jupiter. The earliest satisfactory photographs of the wonder ful planet Saturn, with his accompanying system of rings, were obtained by M. M. Henri, at the Paris Observatory, in 1885.
Mars has always proved a fascinating object on account of its being our nearest celestial neighbour after the moon, and, further, it is thought that the conditions of its surface are somewhat similar to our own—that is, there seems good reason for believing that land and water are present in separated masses as on our earth, and the presence of ice is also very probable. The photographs of Mars show very clearly that the surface of the planet is irregularly divided into dusky and light patches, and at the two diametrically opposite points corre sponding to the axes of rotation, there are small oval patches of intensely white material, which are generally considered to be the snow caps at the extremities of the poles of the planet, corresponding to our Arctic and Antarctic regions of perpetual ice and snow. This view is rendered more feasible by the further fact that these white polar caps are found to vary regularly in extent, getting smaller and smaller as the summer season on the planet approaches, and gradually increasing again on the advent of the Martian winter.