Man's physical universe

xanabras

64 THE UNIVERSE A VAST SYSTEM OF PARTS

by use of a compound lens which is made up of a number of different

lenses ground to different shapes.

The best magnification that GaHleo could obtain with a simple

objective was never greater than one hundredfold, while modern

objective lenses permit good images 25,000 times this value.

Compound lenses increase the loss of light both by absorption and

and, of course, the larger the lens the thicker it becomes, and

reflection ;

the greater is the amount of light absorbed.

Most of the objections to larger refracting telescopes could be dealt

with satisfactorily today; but larger telescopes would be very unwieldy,

and the cost of construction would be prohibitive. Refractors are the

best type of telescopes as far as smaller instruments are concerned.

Mirror

Eye

Fig. 6. The principle of the reflector. The diagonal mirror is relatively much

smaller than shown in the diagram.

Larger telescopes employ concave mirrors which reflect the light rather

than refract it as the refracting object glasses do, and thus less light

is lost by absorption. All of the colors are reflected at the same angle

so that there is no chromatic aberration in reflecting telescopes. The

spherical aberrations are more pronounced in reflecting telescopes than

in refracting telescopes, but they are partially overcome by proper

grinding of the surface of the mirror to a concave paraboloid shape.

The reflecting telescope is never as good as a refracting telescope for

photographing large areas of the sky because of this spherical aberration.

It is, however, better for photographing small areas because it is

achromatic.

The construction of reflecting telescopes is much simpler than that

of refracting telescopes because only one surface has to be shaped,

rather than four or more surfaces, in the compound lens, for example,

and the color or transparency of the glass is of no importance because

the light does not pass through it. A mirror m'ay be supported from

the back, while a refracting lens has to be supported at the edges.

It should be recalled that magnification is controlled by the eyepieces.

Why, then, should larger objectives be used? The objective