The astronomical telescope is the most simple construction of a telescope, composed of convex lenses only, of which there are but two essentially necessary, though a third is sometimes added to the eye-piece for the purpose of enlarging the field of view. Its construction will be easily understood from a description of the following figure. Its two essential parts are, an object-glass AD, and an eye-glass EY, so combined in a tube that the focus F of the object-glass is exactly coincident with the focus of the eye-glass. Let OB (fig. 44.) represent a distant object, from which rays nearly parallel proceed to the object-lens AD. The rays passing through this lens will cross at F, and form an image of the object at IM. This image forms as it were an object to the eye-glass EY, which is of a short focal distance, and the eye is thus enabled to contemplate the object as if it were brought much nearer than it is in reality. For the rays, which after crossing proceed in a divergent state, fall upon the lens EY, as if they proceeded from a real object situated at F. All that is effected therefore, by such a telescope is, to form an image of a distant object by means of the object-lens, and then to give the eye such assistance as is necessary for viewing that image as near as possible, so that the angle it shall subtend at the eye shall be very large compared with the angle which the object itself would subtend in the same situation. [Illustration: _figure 44._] Here it may be expedient to explain, 1. how this arrangement of glasses shows distant objects distinctly, and 2. the reason why objects appear magnified when seen through it. As to the first particular, it may be proved as follows:--The rays OA and BD, which are parallel before they fall upon the object-glass, are by this glass refracted and united at its focus: In order, then, to distinct vision, the eye-glass must re-establish the parallelism of the rays,--which is effected by placing the eye-glass so that its focus may be at F, and consequently the rays will proceed from it parallel to each other and fall upon the eye in that direction. For distinct vision is produced by _parallel_ rays. 2. The reason why the object appears magnified will appear, if we consider that, if the eye viewed the object from the centre of the object-glass, it would see it under the angle OCB; let OC and BC then be produced to the focus of the glass, they will then limit the image IM formed in the focus. If then, two parallel rays are supposed to proceed to the eye-glass EY, they will be converged to its focus H, and the eye will see the image under the angle EHY. The apparent magnitude of the object, therefore, as seen by the naked eye, is to the magnitude of the image as seen through the telescope, as OCB to EHY, or as the distance CF to the distance FG, in other words, _as the focal length of the object-glass to that of the eye-glass_.