II

REFRACTING TELESCOPES.

It has been already mentioned that the principal difference between various refracting telescopes consists in the form of the eye-piece. If the latter is concave, and placed within the focus of the object-glass, erect vision will be obtained: but if convex, and without the focus, vision will be inverted, and therefore additional lenses must be employed when viewing terrestrial objects. The simplest form of achromatic telescope for terrestrial purposes, in which an erect image is of course necessary, will therefore consist of a convex object-glass and a concave eye-piece, and these glasses constitute what is known as the Galilean telescope.

THE GALILEAN TELESCOPE.



Fig. i.

This was the earliest telescope devised, and therefore some of the grandest astronomical discoveries have been made with it. But it possesses several defects which it is not possible to perfectly remedy, the principal being, that the full illuminating power of the object-glass is not used, and that any defect in the material of which the obj ect-glass is made, such as air bubbles, scratches, &c., are rendered visible in the field of view. Proctor has well compared the illumination of the Galilean telescope to that of a view seen through the larger end of a cone of paper held in one position. If an observer move his eye round the inner edge of the larger end of the cone, he will see the confines of the same extent of view he would at once see should he look through the small end. The latter view is that seen in the ordinary refracting telescope, as in it the extent of the field depends on the eye-piece, but in the Galilean, on the aperture of the object-glass. A stop placed in front of the object-glass of the former occasions loss of light, but it diminishes the field of the latter. The centre of this field will also always be brighter than the edge, a most serious defect, and one that is prejudicial to the use of the Galilean telescope as an astronomical instrument.

THE PERSPECTIVE AND OPERA-GLASSES.

A perspective glass is a short focus Galilean telescope with achromatic lenses; it is now seldom used singly, but in pairs, these forming what are known as opera, race, marine, and field-glasses. Should it be desired to view as large a space as possible at one time an opera* glass should be used. But when magnifying power is required, as in the identification of a small distant object, a race, field, or

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