COMETS, PHOTOGRAPHING The first attempt to portray the form of a comet was in the case of Donati's comet of 1858, but the results were very imperfect, owing to the photographic processes being then in their infancy. The first useful photographic cometary records are of Tebbutt's comet of 1881. The gelatine dry plate had been introduced, and with its increased rapidity, compared with the old collodion plates, the problem was Much less formidable. Further improvements were made by the employment of large-aperture telescopes, chiefly of the reflector class. Inasmuch as the comet is generally moving very rapidly in a special orbit of its own, irrespective of the earth's direction of rotation, the usual equatorial tele scope is of little use unless special arrangements are made. To obviate the difficulty various schemes have been devised. The most success ful, and the only one we need mention in a prac tical treatise, is that employed by Prof. F. Barnard, of the Yerkes Observatory, near Chicago. He first makes a preliminary obser vation to determine the rate of motion of the cometary nucleus, and its direction as pro jected on the sky. Then, attached to the eye piece of his telescope, with which he follows the comet nucleus during exposure of the plate, he provides a fine spider thread fixed on a mov able frame actuated by a delicate micrometer screw. If, now, he knows how far the comet will move on the ground glass of his camera in, say, a second, he has only to move this adjust able cross-wire, set in the direction of the comet's motion, by the same amount, and then by keeping the comet nucleus continually bisected by the cross-wires, its image will of necessity be kept exactly on the same portion of the photo graphic plate. It is a similar problem, but some
what more delicate, to that of taking a series of photographs of n moving object with a kine matograph camera : the whole apparatus is usually traversed by means of a screw-and-worm gear.
For recording the whole phenomena attend ing the passage of a comet, probably the most useful instrument is a wide-angle camera attached to a perfectly rigid form of equatorial mounting. Needless to say, the better the lens that is avail able the better will be the photographs. The modern wide-angle anastigmat is the ideal instrument, and as in these cases it is an object of definite area that has to be portrayed, the greater the ratio of aperture to focal length the shorter will be the time of exposure necessary to obtain a satisfactory image, and in consequence the risks of failure due to vibration or bad weather will be minimised in proportion.
The plates used should certainly be backed and panchromatic, as a considerable proportion of the cometary light is green and yellowish-green, and this is all ineffective if ordinary plates, sen sitive only to the blue and violet, are employed.
Development should be very carefully per formed, as in general the range of gradation will be very great, varying from the intensely brilliant nucleus to the filmy streamers constitut ing the delicate tail. The developer may be pyro soda, rodinal, metol, or metol-hydroquinone, but the developers giving excessive density with out the full scale of detail should be avoided.