InSilliman's Journal for 1881 Mr E. S. Holden, after quoting observations to a like effect by Sir W. Herschel, gives details of some observations recently made with a large telescope at the Washburn Observatory, from which it appears that distant objects on a dark but clear night can be seen with the telescope long after they have ceased to be visible with the naked eye. He concludes, “It appears to me that this confirmation of Herschel's experiments is important, and worth the attention of physicists. So far as I know there is no satisfactory explanation of the action of the ordinary Night glass, nor of the similar effect when large apertures are used.”

It is a well-known principle that no optical combination can increase what is called the ‘apparent brightness’ of a distant object, and indeed that in consequence of the inevitable loss of light by absorption and reflection the ‘apparent brightness’ is necessarily diminished by every form of telescope. Having full confidence in this principle, I was precluded from seeking the explanation of the advantage in any peculiar action of the telescope, and was driven to the conclusion that the question was one of apparent magnitude only,—that a large area of given small ‘apparent brightness’ must be visible against a dark ground when a small area would not be visible. The experiment was tried in the simplest possible manner by cutting crosses of various sizes out of a piece of white paper and arranging them in a dark room against a black back-ground. A feeble light proceeded from a nearly turned-out gas-flame. The result proved that the visibility was a question of apparent magnitude to a greater extent than I had believed possible.