Page:Optics.djvu/151

 It appears from this, that to make a telescope of great magnifying power, the object glass should be of considerable focal length or small power, and the eye glass, on the contrary, very powerful.

In consequence, before the discovery of reflecting telescopes, and achromatic combinations of lenses, astronomers were obliged to use instruments of so great a length as to be hardly manageable, till an ingenious mechanic, named Hartsocker, divided the object glass from the eye piece, fixing the former with its frame on the roof of a house, or on a high pole. Huyghens, improving upon Hartsocker's plans, made use of the apparatus represented in Fig. 177.

When merely common lenses are used in this telescope, the eye glass is limited in power by the necessary confusion in the first image from the aberrations, which become very sensible when that image is much magnified, particularly those arising from the unequal refraction of the light.

For this reason, all good telescopes of this kind are made with an achromatic compound lens for an object glass.

The spherical aberration may be lessened, preserving the magnifying power, by using a very weak object glass, assisted by another, called a field-glass, as in Fig. 178.

167. The field of view of a telescope is the lateral extent of prospect it affords in one position, the greater or less portion of the heavens it makes visible at once. To determine this we have only to find the extreme direction in which a ray can pass from one lens through the other: for this we must join the corresponding extremities of the object and eye glass, Fig. 179; the lines, will thus bound the visible image , and the field of view is measured by the angle.