Up: Glassmaking
Lens Story: 5 of 28
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SYSTEM OF LENSES IN ASTRONOMICAL TELESCOPE
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beautiful Milky Way. As a result, this
belt of silvery light was resolved into a myriad of stars too faint to
be distinguished without optical aid, and at such measureless distances
that they literally seemed to rub elbows with each other.
And yet this galaxy of stars represents
SYSTEM OF LENSES
AND PRISMS IN
BINOCULARS
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innumerable blazing suns like our own,
separated from each other by millions and millions of miles. A still
greater discovery was that of the four moons of Jupiter. Here was a
miniature solar system, with its central sun and family of revolving
planets. In quick succession Galileo discovered that the planet Venus
passes through phases as does our moon, he observed the rings of Saturn,
studied the surface of the moon, and, by observation of sun spots,
proved the rotation of the sun on its axis.
The Principle of Lenses
The principle of Galileo's telescope is
preserved in the common opera glass. It is of the refracting type,
as are all lens telescopes, and to understand the production of images
with it we must first become acquainted with the meaning of
refraction. Everyone has observed the apparent
bending at the water line of an oar looked at obliquely or the
misplacement of a line of type when viewed through a thick piece of
plate glass. These effects and many other similar ones are due to the
bending of light rays as they pass from a medium of one density to a
medium of greater or less density. In passing from greater density to
less density the ray is bent away from the perpendicular at the point
of incidence, but in passing from a less dense medium to a more dense
one it is bent toward the perpendicular. Therefore a ray of light from
any part of the oar beneath the water in passing into the air
SEVENTY-TWO-INCH REFLECTING TELESCOPE
Dominion Astrophysical Observatory,
Victoria, B.C.
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bends away from the perpendicular at the
water line. The point from which the light proceeds seems to be in the
direction of the refracted ray and consequently above its true position.
Now, it is in this power of glass similarly to bend light rays that its
optical properties lie. A figure on page 6 shows the bending of a ray
of light in passing through a triangular glass prism. A double convex
lens with which images are produced is in reality two triangular prisms
placed base to base.
Lenses are able to produce two kinds of
images—real and virtual. A real image is one that may be focused
upon a screen and is produced by the actual meeting of the refracted rays
of light at the position in which the image appears to be. Real images
are always inverted with respect to the object. A magic lantern picture
is an illustration of a real image. A virtual image is not formed by the
actual focusing of rays of light but by diverging rays which would meet,
only if produced in the opposite direction. A virtual image cannot be
caught on a screen
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