much easier to construct a very large mirror than to construct a very
large lens; it is also cheaper. A mirror is more likely to get out of
order than is a lens, however, and any irregularity in the shape of a
mirror produces a greater distorting effect than in a lens. A refractor
is also more convenient to handle than is a reflector. For these reasons
great refractors are still made, but the largest of them, the great
Yerkes' refractor, is much smaller than the greatest reflector, the one
on Mount Wilson, California. The lens of the Yerkes' refractor measures
three feet four inches in diameter, whereas the Mount Wilson reflector
has a diameter of no less than eight feet four inches.

[Illustration: THE YERKES 40-INCH REFRACTOR

(The largest _refracting_ telescope in the world. Its big lens weighs
1,000 pounds, and its mammoth tube, which is 62 feet long, weighs about
12,000 pounds. The parts to be moved weigh approximately 22 tons.

The great _100-inch reflector_ of the Mount Wilson reflecting
telescope--the largest _reflecting_ instrument in the world--weighs
nearly 9,000 pounds and the moving parts of the telescope weigh about
100 tons.

The new _72-inch reflector_ at the Dominion Astrophysical Observatory,
near Victoria, B. C., weighs nearly 4,500 pounds, and the moving parts
about 35 tons.)]

[Illustration: _Photo: H. J. Shepstone._

THE DOUBLE-SLIDE PLATE HOLDER ON YERKES 40-INCH REFRACTING TELESCOPE

The smaller telescope at the top of the picture acts as a "finder"; the
field of view of the large telescope is so restricted that it is
difficult to recognise, as it were, the part of the heavens being
surveyed. The smaller telescope takes in a larger area and enables the
precise object to be examined to be easily selected.]

[Illustration: MODERN DIRECT-READING SPECTROSCOPE

(_By A. Hilger, Ltd._)

The light is brought through one telescope, is split up by the prism,
and the resulting spectrum is observed through the other telescope.]

But there is a device whereby the power of these giant instruments,
great as it is, can be still further heightened. That device is the
simple one of allowing the photographic plate to take the place of the
human eye. Nowadays an astronomer seldom spends the night with his eye
glued to the great telescope. He puts a photographic plate there. The
photographic plate has this advantage over the eye, that it builds up
impressions. However long we stare at an object too faint