This historic book may have numerous typos and missing text.
Purchasers can download a free scanned copy of the original book
(without typos) from the publisher. Not indexed. Not illustrated.
1894 Excerpt: ...is the most bent of all, and therefore has its
position farthest off from the straight line. The remaining
colours, such as orange, yellow, green, blue, have each one a
particular position between, dictated by the degree in which it is
bent out of the direct line. So we say that violet is 'the most
refrangible, ' and that red is ' the least refrangible.' This
breaking up of a ray of light may be done in another and somewhat
different mode from that just described. Instead of using a prism,
the ray of light may be allowed to fall, slanting, upon a number of
very fine lines, drawn so close together that the distance between
any two of the lines is small compared with a light-wave. A beam of
light, allowed to pass through or reflected from the surface, will
again be broken up and spread about, as after passing through a
prism. Lines are ruled on glass or metal for this purpose, so fine
and so close together as to be only the twentythousandeth part of
an inch apart. Thus, 20,000 lines can be crowded upon a square
inch, --no easy matter to accomplish, even with all modern
appliances for delicate work. Months may be spent in the making of
a screw for the ruling-engine; and a year may pass in fruitless
searching for a diamond point which will exactly do for such a
task. Then, at the best, when all is prepared, five days and nights
are occupied in the actual ruling of a six-inch grating, with
20,000 lines to the inch. To rule only 14,000 lines to the inch is,
by comparison, an easy matter. Moreover, a glass grating is much
harder to rule than a metal one, for in that case the diamond point
is given to perpetually breaking down. Verily, the pursuit of
knowledge in Astronomy has to be made in the face of obstacles. A
ruled glass or metal of this description is known as.
|Country of origin:
||246 x 189 x 4mm (L x W x T)
||Paperback - Trade
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