Man's physical universe

xanabras

62 THE UNIVERSE A VAST SYSTEM OF PARTS

to the naked eye; a five-inch object glass has about 400 times the area

of the pupil of the eye (which is about | inch in diameter) and can

collect about 400 times as much light.

The largest refractor in the world is the Yerkes telescope at Lake

Geneva, Wisconsin. The Yerkes 40-inch telescope will collect about

Fig. 4. The 40-inch tck-scoiic al ihe

Yerkes Observatory with rising floor at

its lowest position. (Photograph from

the Yerkes Observatory, reprinted by

permission of the Chicago University

Press.)

edged, blurred appearance to dis-

The unaided eye can observe that the

tant trees and other objects.

25,600 times as much light as the

eye and thus render visible

that are

stars

25,600 times too distant

to be visible to the naked eye.

The next largest refracting telescope

is the 36-inch Lick telescope

at the Lick Observatory on Mount

Hamilton near San Jose, California.

James Lick bequeathed three million

dollars to build the telescope

and observatory, which was completed

in 1888. The object glass

for this telescope was cast in

France and polished in America.

Just as light is bent as it passes

from air through glass, so it is bent

by passing from less dense into

more dense air (cold or hot). Variations

in

the moisture content of

the air likewise cause bending of

the light. Everyone has observed

how the air seems to quiver over a

hot surface, lending a soft, feathery-

stars near the horizon seem to flicker because of the optical aberrations

produced by the varying strata of air. To reduce this error as far as

possible, telescopes are located, whenever possible, on mountains whose

locations have been selected after a great deal of investigation. Another

important point to consider in location is to place the telescope where

light from large cities will not interfere and where the least number of

cloudy days will be experienced.

A good telescope cannot be kept in a heated dome in winter, because

the warm air escaping through the opening will produce a wavering of

air which makes observations impossible, while variations in the

density and curvature of the objective lens due to temperature variations

will produce even greater errors.