OPTICAL INSTRUMENTS.
In most optical instruments, lenses are used for the purpose of
gathering a large number of rays of light and altering the apparent
direction of the rays so that an enlarged picture may be presented to
the eye. In order to accomplish this, it is necessary that the rays of
light be bent or refracted. This refraction, we have already seen, is
always accompanied by a dispersion which causes the light to be
dissolved into its original colors more or less. This has been
illustrated by means of prisms.
[Illustration: FIGURE 93.]
[Illustration: FIGURE 94.]
Wherever a single lens is used, the light around the edges of the
illuminated space is always more or less colored, varying with the
illuminant used. Such coloring is most noticeable along the edges of
projected pictures but it also exists, though to a less noticeable
degree, over the whole field, showing least in the center.
The light which is thus refracted and dispersed by one prism may be
gathered again by another, as shown in Figure 93, but the light rays
after passing through the second prism will be exactly parallel to the
ray striking the first. The light coming out of the second prism will
appear white but it will be impossible either to enlarge or diminish the
size of a picture in this way; hence lenses, corrected to give white
light in this manner, would be of no use.
Fortunately it has been found that, with different kinds of glass, the
ratio of refraction to dispersion is different; and by combining two
pieces of glass of different nature, it is possible to recombine the
colors without causing the emergent ray to become parallel to the
incident ray. Consider, for instance, Figure 94 in which we have drawn a
prism made up of two different kinds of glass. If the right half were of
a glass having the same index of refraction as the left, for the red
rays for instance, these would continue through both in a straight line.
If the dispersion were less in the right half, i.e., if the violet rays
were refracted less--sufficiently less to cause them to approach the
red--they would meet the latter at some point outside of the prism and
combine into white light again, thus eliminating the colors ordinarily
visible through single glass lenses.
[Illustration: FIGURE 95.]
Whenever it is necessary to project especially good pictures upon a
screen, lenses corrected in some such manner as outlined above are
always used and the lenses are often combined as shown in Figure 95. In
this figure, _R_ indicates the line, through the principal axis, at
which the red rays refracted by lens 1 alone would strike; and _V_, the
line where the violet rays would be projected. The addition of lens 2
brings the red and violet together again at _W_. A combination of two
such lenses, placed the proper distance apart and the surfaces properly
proportioned, may be made to combine any two of the colors of the
spectrum. Hence even with these corrected lenses there is always some
coloring on the screen although it is hardly noticeable.
[Illustration: FIGURE 96.]
[Illustration: FIGURE 97.]
[Illustration: FIGURE 98.]
Figures 96, 97, and 98 are drawings showing the manner in which
objective lenses are usually made up. The types at the right and left
are used for camera work, while the one shown in the center is used
mostly for moving picture and stereopticon projection. The end having
the separate lenses is turned towards the light. Those shown in contact
are glued together by the use of Canadian balsam.
[Illustration: FIGURE 99.]
The optical system of the ordinary telescope is shown in Figure 99.
Light from the distant object _A_ is gathered by the large lens _B_ and
an image is formed as indicated by the small arrow. This image acts as
the object to lens _C_ and is projected to lens _D_ where the rays of
light are strongly refracted, entering the eye by angles which cause an
enlarged view of the object at _E_, as indicated. There must of course
be some means by which the lenses may be adjusted to each other for
focusing.
[Illustration: FIGURE 100.]
The arrangement of the opera glass in Figure 100 is quite different from
the above because of the reduced size of the instrument and for the
reason that an erect picture is desired, whereas the telescope above
gives an inverted one. The principal difference between the two is in
the eyepiece. In the opera glass this is a concave lens while in the
other it is a convex lens. In this case it is necessary to have the eye
very close to the lens to catch the rays of light. The opera glass, as
well as the telescope, must be provided with means of varying the
distance between the lenses according to the distance of the object
viewed, for the purpose of focusing. In some telescopes and also opera
glasses, prisms are used for the purpose of obtaining erect images.
Figure 101 will show how the rays of light entering a prism are
reflected and the image reversed thereby.
[Illustration: FIGURE 101.]
Figure 102 is an explanation of the reflecting stereoscope. Let the
black circles represent the eyes of the observer and let _M_ represent
two mirrors placed as shown. If two pictures taken by a stereoscopic
camera are placed as indicated by the arrows at the right and the left,
they will appear superimposed in the position of the arrow in the rear.
[Illustration: FIGURE 102.]
[Illustration: FIGURE 103.]
The refracting stereoscope is the one mostly used and the plan of it is
shown in Figure 103. Pictures for use with stereoscopes are taken by
special cameras provided with two lenses placed about as far apart as
the human eyes and mounted together. Stereoscopic effects may, however,
be produced even without this precaution and it is possible to obtain
some queer results by combining certain pictures.
In the so-called “Camera Lucida”, prisms of the type shown in Figures
104 and 105 are used. At the left is a combination lens and reflecting
prism which gives an erect image, and Figure 105 is a prism also
designed to give erect images. Such instruments are used for sketching.
They may be made to throw an image upon a small screen where its lines
may be traced out by the artist.
[Illustration: FIGURE 104.]
[Illustration: FIGURE 105.]
The most important optical instrument with which we have to deal is the
_projecting arc lamp_ and its optical system. The passage of light
through this system of lenses is altogether different from that passing
through a camera lens for instance. In a camera lens the picture is
formed upon the ground glass screen by the light reflected from the
object. From any single point of the object, rays of light strike all
parts of the lens, pass through it, and are recombined or focused at
some point behind it. Under these conditions, focus can be obtained only
at a certain distance behind the lens, this distance varying with the
distance of the object whose light is being received by the lens.
[Illustration: FIGURE 106.]
[Illustration: FIGURE 107.]
[Illustration: FIGURE 108.]
With our projecting lens we have no reflected light leaving the object
in all directions, but instead we have rays of light having definite
directions. This can be seen from Figure 106. The light used must come
from a point source, the smaller and the more intense it is the better.
This light is gathered by condensers, as shown at _C_, which are so
arranged as to focus the light in the center of the objective lens _D_.
In the moving-picture machine the light, before reaching the objective
lens, is passed through the film as indicated, the arrows representing
sections of film. The picture projected in this manner can be made to
appear upon a screen in front of the object lenses at any distance, but
the farther away it is, the larger it will be and the less bright the
illumination of it. A picture projected in this manner is always
inverted, and, in order to have it appear right side up, it must be
placed in position upside down. Figures 107 and 108 show arrangement of
lenses frequently used as condensers.