Fig. 108.—Microscope as Ocular.
Fig. 109.—“Davon” Instrument.
Quite analogous to the eyepiece microscope is the so-called “Davon” micro-telescope. Originally developed as an attachment for the substage of a microscope to give large images of objects at a little distance it has grown also into a separate hand telescope, monocular or binocular, for general purposes. The attachment thus developed is shown complete in Fig. 109. D is merely a well corrected objective set in a mount provided with ample stops. The image is viewed by an ordinary microscope or special eyepiece microscope A, as the case may be, furnished with rack focussing at A′ and assembled with the objective by means of the carefully centered coupling C.
It furnishes a compact and powerful instrument, very suitable for terrestrial or minor astronomical uses, like the Tolles’ short-focus hand telescopes already mentioned. When properly designed telescopes of this sort give nearly the field of prism glasses, weigh much less and lose far less light for the same effective power and aperture. They also have under fairly high powers rather the advantage in the matter of definition, other things being equal.
[CHAPTER VII]
HAND TELESCOPES AND BINOCULARS
The hand telescope finds comparatively little use in observing celestial bodies. It is usually quite too small for any except very limited applications, and cannot be given sufficient power without being difficult to keep steady except by the aid of a fixed mounting. Still, for certain work, especially the observation of variable stars, it finds useful purpose if sufficiently compact and of good light-gathering power.