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 an indispensable adjunct in the observatory, and we are every day learning more and more of what it can do for us. The chemical eye is often more sensitive than the human eye; it is always more patient. It will display for us a magnificent nebula like that which surrounds the Pleiades, and which is wholly invisible to the unaided eye, and only to be seen with the telescope under very special conditions not often realised. Naturally, Huggins has discussed at length the applications of photography. It would be impossible for him not to have mentioned that photograph obtained by the late Dr. Roberts of the great nebula in Andromeda, which was produced by exposing a highly sensitive plate for four hours in the focus of a powerful reflector. The result has been to produce a picture which has been said, and I believe with truth, to be the most suggestive representation of any celestial object that has ever been obtained. Features which had been dimly traced in the nebula when visually examined in powerful telescopes are now seen to be parts of an organic whole, visible on the photograph, though not otherwise discernible by the keenest sense.

Such a study of this great nebula was all the more acceptable because it is one of the most baffling of these objects. It is bright enough to be perceived by the unaided eye, and it might have been expected that so striking a celestial structure ought by this time to have disclosed its character either as a distant cluster of stars, or as a truly gaseous object. Herschel long ago called it one of the least resolvable of the nebulæ, but yet it does not appear to possess a spectrum similar to that of the gaseous nebulæ of which we have been speaking. The character