Page:Optics.djvu/150

 the latter instrument is used to examine an object placed near it, which is not distinctly visible, on account of its minuteness, whereas the former is calculated to assist the eye in viewing objects, which look small or indistinct, only on account of their distance.

The telescope substitutes for a distant object an image at any distance required, which subtends at the eye a much larger angle than the object, and from which more light proceeds, than the eye could receive from that object. The manner in which this is effected in this instance, is as follows.

By means of a convex lens $A$, called the object glass, there is produced an inverted image of a distant object, which image is, of course, at the principal focus of the glass, and has collected on it nearly all the light which, proceeding from the object, falls on the surface of the object glass.

This image is viewed and magnified by means of an eye glass, another convex lens, which is placed so that the image is at its focus, or a little within it, in which latter case, a second image (though not a real one,) is produced at the most convenient distance for correct vision.

It is evident then, that if the eye glass be a powerful lens, which it always is in practice, the final image subtends a much larger angle at the eye which is placed close to the eye glass, than the object does, and it is brighter, for the eye receives in the one case all the light which falls on the object glass, in the other, only as much of that coming from the object as can pass through the pupil of the eye.

166. If the foci of the two lenses coincide, the magnifying power of this instrument is expressed by $AF⁄Bf$, or $F⁄F′$, if $F$, $F′$ represent the focal lengths of the object and eye glass. If the final image be brought to the nearest distance of distinct vision ($c$), it is greater in the proportion of $c⁄F′+1$ to $c⁄F′$, (see compound microscope,) and therefore its value is