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316 THE ADVANCE OF PHOTOGRAPHY
object with a long tail. Great changes in the structure of its tail were observed on September 16, October 1, and October 16, and these were accompanied with great fluctuations in the brilliancy of the comet.
At all the principal observatories series of photographs were taken, and from these many important facts with respect to the nature of these heavenly bodies have been learned. These photographs show clearly that the process of tail formation is of an intermittent and not continuous nature. In no previous comet has the motion of the matter forming the tail been so clearly traced.
The behaviour of the nucleus, and the phases through which the comet passed as parabolic envelopes of matter appeared on the side towards the sun ; the sweeping round of the matter to produce the beautiful fan-shaped tail, and finally the break-up of the tail into fleecy masses, can all be clearly observed in the photographic records. Professor E. E. Barnard l has combined a number of the photographs taken at intervals of one hour or so, and has thus obtained beautiful stereoscopic effects.
The comet of course appears as a brilliant object suspended in space among the stars ; but while he believes that a great many of the stereoscopic details give incorrect ideas of what was taking place at the time, yet he considers much can be learned of the general structure of such bodies by this means.
Jupiter's Eighth Satellite. — Lastly, the brilliant and patient work of Mr Melotte of the Royal Observatory, Greenwich, as a result of which he discovered the eighth satellite of Jupiter, is still another success due to photography. Let us for a moment try to understand the difficulty of discovering the existence of such a small heavenly body as this satellite, which is equivalent in brightness to a star of the seventeenth magnitude.
Stars are classed in magnitude according to their
1 See R.A.S. Notices, June 1909.