Motography (Apr-Dec 1911)

April, 1911. MOTOGRAPHY 39 Problems of the Projecting Room By William X. Braun THE A, B, C OF OPTICS AND LENSES. THE science of optics is a dark mystery to the average operator, and although it is not necessary for him to be deeply versed in the study, he should know enough about it to be able to handle his projection intelligently. The writer realizes the objection of many operators to technical terms, so he will endeavor to explain them whenever used in order that he may be thoroughly understood. Lenses have the property of causing the rays of light which pass through them to converge or come together as in F Fig. 2; or to diverge or grow farther apart as in LA, Fig. 5. Lenses are made either of crown glass, which is free from lead, or of flint glass, which contains lead and is more refractive than crown glass. The arrangement and kinds of lenses used in moving picture machines of today are the same that have been in use in the stereopticon for many years. When a ray of light strikes a lense one of two things may happen. It will either be transmitted, that is pass directly through as ray MF, Fig. 2, or be refracted as ray LF, Fig. 2. There are six kinds of lenses used in projection work — classifying them according to their curvatures. Cross sections of these are Fig. lr shown in Fig. 1. A is 2. double convex, B is a plano-convex, C is a converging concavo-convex, D is a double concave, £ is a plano-concave, and F is a diverging concavo-convex. C and F are sometimes called meniscus lenses from their resemblance to the crescent-shaped moon. The sides of the first three can be seen to converge or come to a point, while the last three diverge or their sides spread farther apart. In lenses whose surfaces are spherical the centers for these surfaces are called the centers of curvature, and the line connecting them the principal axis. As all surfaces of lenses are made up of an infinite number of flat surfaces, each ray of light on_ passing through the lens is refracted or bent perpendicular to this surface. It can be seen that the rays passing through A, B, or C, the converging lenses, are converged or bent in, while those passing through D, E, or F are diverged or bent outward. We now come to the question of foci of lenses. The focus of a lens is the point on the principal axis where the refracted rays or their prolongations meet, and is marked F in all of the diagrams. First consider the focus of the double 'convex lens shown in Fig. 2. If the rays of light L are parallel to the principal axis MN, the rays on passing through the lens will be refracted, meeting the axis at F. This point is called the real focus and the distance FA the focal length. This point varies with lenses of various curvatures and the refractive powers of the lens. For ordinary crown glass this point coincides very closely with +1">e center of curvature. Now, if we take the source of light at a point, as that or toe arc lamp, but place it outside of the focus as at L Fig. 3, the rays after passing through the lens will come to a point / on the principal axis, the Fig. 2. Fig. 4. conjugate focus of the Point L. If the light is placed at / the rays will meet at L. This is the principle of the condenser lens. As the light is moved toward the lens, C moves further away ; when L reaches F the rays after leaving the lens will be parallel to the principal axis as at L Fig. 2. If the light is placed at L as in Fig. 4, between F and the lens the rays of light on leaving the lens will spread out. If the rays are prolonged they will meet the at / Fig 4. This point is called the virtual focus of the point L. This is the principle of the front set of lens in the projection tube. To determine the real focus of a convex lens place the lens so that the rays of light falling on it are parallel to the principal axis. Allow the rays as they emerge from the lens to fall on a glass. The place where the rays converge to a point can easily be seen. This is the focus. The distance from this point to the lens will be the focal length. In diverging lens such as D, E, and F, Fig. 1, there are only virtual foci at whatever distance the object. In Fig. 5 the rays of light L are parallel to the principal axis FI, and on emerging the rays are refracted outward to A and B. If the rays are pro Fig. 3. longed back through the lens they will meet at F. This point is the virtual focus. To determine the focal length of a concave lens the face of it is covered with any opaque substance, such as lampblack. Two small holes, a and b, Fig. 5, are left open, both being the same distance from the axis. The two rays of light through a and b are received at A and B on the screen or wall P. Move the