Figure 49.7: A compound microscope. (Ref. [3])essentially enlarges the image produced by the objective lens. In most modern microscopes, there are several
objective lenses mounted on a rotating platform, as well as interchangeable eyepieces, so that the user can
select an appropriate combination for the desired magnification.
The microscope is often used in biology to observe cells and protozoa.
49.6 The Telescope
Atelescope(from the Greek , “far”, and
o!, “see”) is an instrument designed to observe far-away
objects. A small hand-held telescope is called aspyglassormonocular; a pair of such small telescopes
mounted side-by-side provide stereo vision and are calledbinoculars.
Larger telescopes are used for astronomical observations. Astronomical telescopes are of one of two
types:
•Arefracting telescopeis made of lenses: a largeobjective lens, and a smallereyepiece.•Areflecting telescopeconsists of one or more curved lenses in place of the objective lens; an eyepiece
lens creates the final image.Refracting astronomical telescopes were built until around 1900; since then, all large astronomical tele-
scopes have been of the reflecting type. This is because there are a number of problems with refracting
telescopes that are avoided in reflecting telescope designs. First, there is a limit on how large it is practical
to make the objective lens. The lens is made of glass; it is therefore fluid, and will tend to flow. There’s
not much that can prevent this, since the lens can only be supported by the edges. Second, since light must
pass through the lens, it is subject to being scattered by any imperfections (bubbles, etc.) that may be in the
glass, which will cause imperfections in the image. Third, since light has to travel through the lens, there
is a tendency for light to be lost as it travels through the lens, and so very faint objects are difficult to ob-
serve. Fourth, a refracting telescope is subject to chromatic aberration. All of these problems are avoided by
reflecting telescopes.
The largest refracting astronomical telescope still in use is at the Yerkes Observatory in Wisconsin; this
telescope has an objective lens with a 40-inch diameter. In contrast, reflecting telescopes of over 400 inches
diameter have been constructed.