The Aerial is a refracting telescope of the kind we have now described, intended to be used without a tube in a dark night; for the use of a tube is not only to direct the glasses, but to make the place dark where the images are formed. It appears from the preceding table inserted above, that we cannot obtain a high magnifying power, with the common astronomical telescope, without making it of an extreme length, in which case the glasses are not manageable in tubes--which are either too slight and apt to bend, or too heavy and unwieldy if made of wood, iron or other strong materials. The astronomers of the seventeenth century, feeling such inconveniences in making celestial observations with long tubes, contrived a method of using the glasses without tubes. Hartsocker, an eminent optician, contrived to fix them at the top of a tree, a high wall, or the roof of a house; but the celebrated Huygens, who was not only an astronomer, but also an excellent mechanic, made considerable improvements in the method of using an object-glass without a tube. He placed it at the top of a very long pole, having previously enclosed it in a short tube, which was made to turn in all directions by means of a ball and socket. The axis of this tube he could command with a fine silken string, so as to bring it into a line with the axis of another short tube which he held in his hand, and which contained the eye-glass. The following is a more particular description of one of these telescopes. On the top of a long pole or mast _ab_ (fig. 45), is fixed a board moveable up and down in the channel _cd_: _e_ is a perpendicular arm fixed to it, and _ff_ is a transverse board that supports the object glass enclosed in the tube _i_, which is raised or lowered by means of the silk cord _rl_; _gg_ is an endless rope with a weight _h_, by which the apparatus of the object-glass is counterpoised; _kl_ is a stick fastened to the tube _i_; _m_ the ball and socket, by means of which the object-glass is moveable every way: and to keep it steady, there is a weight _n_ suspended by a wire; _l_ is a short wire to which the thread _rl_ is tied; _o_ is the tube which holds the eye-glass; _q_ the stick fixed to this tube, _s_ a leaden bullet, and _t_ a spool to wind the thread on; _u_ is pins for the thread to pass through; _x_ the rest for the observer to lean upon, and _y_ the lantern. Fig. 46 is an apparatus contrived by M. de la Hire for managing the object-glass; but which it would be too tedious particularly to describe. To keep off the dew from the object-glass, it was sometimes included in a pasteboard tube, made of spongy paper, to absorb the humidity of the air. And to find an object more readily, a broad annulus of white pasteboard was put over the tube that carried the eye-glass; upon which the image of the object being painted, an assistant who perceived it, might direct the tube of the eye-glass into its place. [Illustration: _figure 45._] [Illustration: _fig 46._]