It is necessary, in order to distinct vision, that the glasses, particularly the eye-glasses of telescopes be kept perfectly clean, free of damp, dust, or whatever may impede the transmission of the rays of light. But great caution ought to be exercised in the wiping of them, as they are apt to be scratched, or otherwise injured by a rough and incautious mode of cleaning them. They should never be attempted to be wiped unless they really require it; and, in this case, they should be wiped carefully and gently with a piece of new and soft lamb’s-skin leather. If this be not at hand, a piece of fine silk paper, or fine clean linen may be used as a substitute. The lens which requires to be most particularly attended to is the second glass from the eye, or the field-glass; for if any dust or other impediment be found upon this glass, it is always distinctly seen, being magnified by the glass next the eye. The next glass which requires attention is the fourth from the eye, or that which is next the object. Unless the glass next the eye be very dusty, a few small spots or grains of dust are seldom perceptible. The object-glass of an achromatic should seldom be touched, unless damp adheres to it. Care should be taken never to use pocket handkerchiefs or dirty rags for wiping lenses. From the frequent use of such articles, the glasses of seaman’s telescopes get dimmed and scratched in in the course of a few years. If the glasses be exceedingly dirty, and if greasy substances are attached to them, they may be soaked in spirits and water, and afterwards carefully wiped. In replacing the glasses in their socket, care should be taken not to touch the surfaces with the fingers, as they would be dimmed with the perspiration: they should be taken hold of by the edges only, and carefully screwed into the same cells from which they were taken. ON MEGALASCOPES, OR TELESCOPES FOR VIEWING VERY NEAR OBJECTS. It appears to have been almost overlooked by opticians and others, that telescopes may be constructed so as to exhibit a beautiful and minute view of very near objects, and to produce even a microscopic effect, without the least alteration in the _arrangement_ of the lenses of which they are composed. This object is effected simply by making the eye-tube of a telescope of such a length as to be capable of being drawn out 12 or 13 inches beyond the point of distinct vision for distant objects. The telescope is then rendered capable of exhibiting with distinctness all kinds of objects, from the most distant to those which are placed within 3 or 4 feet of the instrument--or not nearer than double the focal distance of the object-glass. Our telescopes, however, are seldom or never fitted with tubes that slide farther than an inch or two beyond the point of distinct vision for distant objects, although a tube of a longer size than usual, or an additional tube would cost but a very trifling expence.