Page:Optics.djvu/153

 169. There is another kind of micrometer used, which may as well be described here while we are on the subject: it is called the divided object glass micrometer, and is, in fact, an object glass divided into two by a plane passing through its axis, and of which the two parts, when placed with their centres not coinciding, act as two separate lenses, (Figs. 184, and 185.) The use of this construction is to make the images of two stars, not far distant from each other, coincide on the axis of the telescope, and to determine the angular distance by observing how much the centers of the half lenses have been displaced, by graduated scales on the edges.

Sometimes the eye glass of a telescope is divided in this manner instead of the object glass, and this is thought by many persons a preferable arrangement.

170. In this the rays refracted through the object glass, and proceeding to form an image at its focus, as in the Astronomical Telescope, are stopped by a concave lens, which, if it have a common focus with the object glass, makes them emerge parallel, or if placed nearer, gives them any degree of divergency suited to the eye.

By this means the first image is made imaginary, and the second, likewise imaginary, is thrown on the opposite side of the eye glass, and is therefore erect.

This is the principle of the opera glass, which has the advantage of being much shorter than the astronomical telescope in which the eye glass is beyond the first image, besides representing objects in their natural erect position.

This was the first telescope invented.

The magnifying power, which in this instrument is commonly very low, is represented as before by the fraction $F⁄F′$.

This telescope might be used as a microscope, but it would require to be lengthened, as the first image would be thrown farther from the object glass than its principal focus; the magnifying power would in this case be increased.