Galileo’s best telescopes magnified only some thirty times, and the views which he thus obtained, must have been similar to those exhibited by the smaller photographs of the moon produced in late years by Mr. De la Rue and now familiar to the scientific public. Of course there is in the natural moon as viewed with a small telescope a vivid brilliancy which no art can imitate, and in photographs especially there is a tendency to exaggeration of the depths of shade in a lunar picture. This arises from the circumstance that various regions of the moon do not impress a chemically sensitized plate as they impress the retina of the eye. Some portions, notably the so-called “seas” of the moon, which to the eye appear but slightly duller than the brighter parts, give off so little _actinic_ light that they appear as nearly black patches upon a photograph, and thus give an undue impression of the relative brightness of various parts of the lunar surface. Doubtless by sufficient exposure of the plate in the camera-telescope the dark patches might be rendered lighter, but in that case the more strongly illuminated portions, which after all are those most desirable to be preserved, would be lost by the effect which photographers understand as “solarization.”
In speaking of a view of the moon with a magnifying power of thirty, it is necessary to bear in mind that the visible features will differ considerably with the diameter of the object-glass of the telescope to which this power is applied. The same details would not be seen alike with the same power upon an object-glass of 10 inches diameter and one of 2 inches. The superior illumination of the image in the former case would bring into view minute details that could not be perceived with the smaller aperture. He who would for curiosity wish to see the moon, or any other object, as Galileo saw it, must use a telescope of the same size and character in all respects as Galileo’s: it will not do to put his magnifying power upon a larger telescope. With large telescopes, and low powers used upon bright objects like the moon, there is a blinding flood of light which tends to contract the pupil of the eye and prevent the passage of the whole of the pencil of rays coming through the eye-piece. Although this last result may be productive of no inconvenience, it is clearly a waste of light, and it points to a rule that the lowest power that a telescope should bear is that which gives a pencil of light equal in diameter to the pupil of the eye under the circumstances of brightness attendant upon the object viewed. In observing faint objects this point assumes more importance, since it is then necessary that all available light should enter the pupil. The thought suggests itself that an artificial enlargement of the pupil, as by a dose of belladonna, might be of assistance in searching for faint objects, such as nebulæ and comets: but we prefer to leave the experiment for those to try who pursue that branch of astronomical observation.