1. _Adjustments requisite to be attended to in the use of telescopes._ When near objects are viewed with a considerable magnifying power, the eye-tube requires to be removed farther from the object-glass than when very distant objects are contemplated. When the telescope is adjusted for an object, 6, 8, or 10 miles distant, a very considerable alteration in the adjustment is requisite in order to see distinctly an object at the distance of two or three hundred yards, especially if the instrument is furnished with a high magnifying power. In this last case, the eye-tube requires to be drawn out to a considerable distance beyond the focus for parallel rays. I have found that, in a telescope which magnifies 70 times, when adjusted for an object at the distance of two miles, the adjustment requires to be altered fully one inch in order to perceive distinctly an object at the distance of two or three hundred yards; that is, the tube must be drawn, in this case, an inch farther from the object-glass, and pushed in the same extent, when we wish to view an object at the distance of two or three miles. These adjustments are made, in pocket perspectives, by gently sliding the eye-tube in or out, by giving it a gentle circular or spiral motion till the object appear distinct. In using telescopes which are held in the hand, the best plan is to draw all the tubes out to their full length, and then, looking at the object, with the left hand supporting the main tube near the object-glass, and the right supporting the eye-tube--gently and gradually push in the eye-piece till distinct vision be obtained. In Gregorian reflecting telescopes this adjustment is made by means of a screw connected with the small speculum; and in large achromatics, by means of a rack and pinion connected with the eye-tube. When the magnifying power of a telescope is comparatively small, the eye-tube requires to be altered only a very little.