CHAPTER XII.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.