Excerpt from Publications of the Astronomical Society of the Pacific, Vol. 6 It is certain that a photograph, properly taken, will Show many features of comets, nebulae and the Milky Way which the eye cannot grasp as well, or even at all. Thus the photo graphic Milky Way must always be different from the visual one, and future researches may well be directed solely to the former. The recent photographs of Comet Swift (april, of Comet b (july, 1893) and of Comet Brooks (october, 1893) taken at the lick Observatory, by Professors barnard and hussey, exhibit several features in the fainter portions of these comets which are not visible to the eye and which have not been seen in earlier comets. Yet it is very possible, and even probable, that these features may have been present in former bright comets. For the tails of comets, then, we shall always prefer photographs to drawings. The case is entirely the same for nebulae. Why should it be different for the Milky Way? Photographs of the Milky Way do, in fact, Show some features which the eye cannot see at all (and hence duplicate negatives should be taken for verification) and they Show very many others which the eye sees, indeed, but not with sufficient sharpness to allow of a satisfactory delineation. The photograph has, however, errors of its own which are of the same general nature as the errors of the human retina and it takes account of one part of the spectrum only.* A considerable part of the luminous background of the Milky Way, as seen with the naked eye, is due to the fact that the eye cannot separate the different stars which really exist in the sky and whose images, therefore, overlap on the retina. Moreover, the eye cannot be rigidly fixed on one area, but leaves it and returns to it continuously; and hence the persistence of vision contributes also to the formation of the luminous back ground which we (apparently) see. A considerable part of this background has, therefore, no real existence, but is a strictly subjective appearance, and must, consequently, be different to different eyes. When a telescope is employed to view the Milky Way, phenomena of the same general nature occur. A telescope has a greater separating power than the eye, and more stars are separately seen; but those which are not individually shown produce the effects just described. The larger the telescope the greater the separating power and the larger the number of stars which form their individual images, and the fewer stars, therefore. About the Publisher Forgotten Books publishes hundreds of thousands of rare and classic books. Find more at www.forgottenbooks.com This book is a reproduction of an important historical work. Forgotten Books uses state-of-the-art technology to digitally reconstruct the work, preserving the original format whilst repairing imperfections present in the aged copy. In rare cases, an imperfection in the original, such as a blemish or missing page, may be replicated in our edition. We do, however, repair the vast majority of imperfections successfully; any imperfections that remain are intentionally left to preserve the state of such historical works.