There are two kinds of telescopes, corresponding to two modes of vision, namely, those which perform their office by _refraction_ through lenses, and those which magnify distant objects by _reflection_ from mirrors. The telescope which is constructed with lenses, produces its effects solely by refracted light, and is called a Dioptric, _or refracting telescope_. The other kind of telescope produces its effects partly by reflection, and partly by refraction, and is composed both of mirrors and lenses; but the mirrors form the principal part of the telescope; and therefore such instruments are denominated _reflecting telescopes_. In this chapter I shall describe the various kinds of _refracting_ telescopes. SECT 1.--THE GALILEAN TELESCOPE. This telescope is named after the celebrated Galileo, who first constructed, and probably _invented_ it in the year 1609. It consists of only two glasses, a _convex_ glass next the object, and a _concave_ next the eye. The convex is called the _object-glass_, and the concave to which the eye is applied, is called the _eye-glass_. Let C (fig. 43.) represent the convex object-glass, presented to any object in the direction DEI, so that the rays fall parallel upon it;--if these rays, after passing through it, were not intercepted by the concave lens K, they would pass on, and cross each other in the focus F, where an inverted image of the object would be formed. But the concave lens K, the virtual focus of which is at F, being interposed, the rays are not suffered to converge to that point, but are made less convergent,[19] and enter the pupil almost parallel, as GH, and are converged by the humours of the eye to their proper foci on the retina. The object, through this telescope, is seen upright, or in its natural position, because the rays are not suffered to come to a focus, so as to form an inverted picture. The concave eye-glass is placed as far within the focus of the object-glass, as is equal to its own virtual focus; and the magnifying power is as the focal length of the object-glass to that of the eye-glass, that is, as CF to BF. Thus, suppose the focus of the object-glass to be 10 inches, and the focus of the eye-glass to be 1 inch, the magnifying power will be 10 times--which is always found by dividing the focal length of the object-glass by that of the eye-glass. The interval between the two glasses, in this case, will be 9 inches, which is the length of the telescope, and the objects seen through it will appear under an angle ten times greater than they do to the naked eye. These propositions might be proved mathematically; but the process is somewhat tedious and intricate, and might not fully be understood by general readers. I shall therefore only mention some of the general properties of this telescope, which is now seldom used, except for the purpose of _opera-glasses_. [Illustration: _figure 43_]