International projectionist (Jan-Dec 1950)

PEOPLE are intrigued by parlor magic and simple phenomena; they like to know the answers to things that they may have observed but do not understand. This human characteristic presents a real opportunity to the man interested in the science of optics, because there are many simple stunts which serve to demonstrate some point of optical significance. Of course, we have constant evidence of nature's optical magic. The stick that seems to bend in a pool of water, the color in the rainbow or sunset, the brilliant gleam of a diamond, the shimmer of heat waves on a highway, the iridescense of oil on water — all are demonstrations of basic optical principles. But let's consider in particular some of the optical parlor tricks that can be performed with readily available accessories. Not one of them is new or Optical FIGURE 1. original. But all are useful as conversational spark plugs. All are worthy of review by our readers. Stereoscopic Vision Few people realize the importance of binocular vision in determining depth or distance. But it's easy to show them. Try asking your friend to hold his arms out with elbows slightly bent. Ask him to close one eye and to try to bring FIGURE 2. the tips of his index fingers together, as in Fig. 1. Most people will miss by a half inch or more on the first try. If that seems too easy, because of muscular co-ordination, here is an alternative. Lay a pencil on a desk or table so that its point extends beyond the edge, as in Fig. 2. Again, with one eye closed, have him try to touch the point with the tip of the finger. Not so easy! The Dominant Eye Point your finger or hold up a pencil so that it is in line with a distant object, as in Fig. 3. Now close one eye, then the other. It will be readily seen that the dominant eye (usually the right) has been used in sighting the distant object. "But I can't line it up," someone will Oddities By MILTON C. WILLIAMSON Bausch & Lomb Optical Company say, "because I seem to see two pencils when I look at a distant object." Then, let him try this. Tear a hole about the size of a nickel in a piece of paper and hold the paper at arm's length, fixing on some distant point. Have him move the paper slowly toward his face until it touches his nose. There's his dominant eye — looking right through the hole. Pinhole Performance Probably the pinhole is the simplest optical device in the world. A competent photographer can use it instead of a lens to obtain an excellent picture. And you can use a pinhole to point out fundamental optical truths. Punch a pinhole in a card and have someone hold it close to his eye. Then ask him to hold his finger or some small object in front of the pinhole. Show him how he has achieved "universal depths of focus" — sharpness near and far at the same time. While we know that some of our professional friends avoid a comparison of the eye and the camera, this phenomenon can best be explained in photographic terms. Make a sketch of Fig. 4. Explain that the top illustration represents a photographic lens at full aperture. Points A and B are "circles of confusion," 1/100" in diameter, between which all images appear sharp. Now "stop down" the lens to a small opening as in the lower il greater range (near to far) become sharp — the depth of focus has been increased. Having demonstrated these principles, describe how a pinhole diaphragm in a camera or before the eye will result in almost universal depth of focus. With a bright light, even a presbyope (farsighted) can read a telephone book through a pinhole. This experiment helps to explain the action of the diaphragm in the human eye. As you know, a person who is mildly presbyopic may be able to read a newspaper at arm's length under ordinary lighting. However, if he holds his paper in direct sunlight, he will find that he can read it with ease at a shorter distance. When the bright light causes the diaphragm to contract, the result is greater depth of focus, which helps to overcome limited accommodation. Now hold a pinhead close to the eye and between the iris and the pinhole. A shadow is> cast upon the retina right side up. The pin appears to the viewer to be upside down, a beautiful demonstration of the remarkable mental and lustration. Points A and B move away from the focal plane. Objects over a FIGURE 4. psychological processes by which retinal images are inverted! The Blind Spot All of our opthalmic friends are familiar with this demonstration, which we are including only by way of reminder. Mark two black dots on a piece of paper, as in Fig. 5. With the left eye closed, hold the card in line with the right eye. Watching the left dot, move the card slowly toward the face and observe there is one position in which the right dot disappears from the indirect field. You know the explanation — the second dot disappears when its image strikes the physiological blind spot. As a more dramatic experiment, try to blot the moon from the sky: "B" represents the moon, "A" the point you must find to bring the image of the moon in coincidence with the blind spot. Fun with Mirrors A person seldom sees himself in a mirror as he really is. When he parts his hair on the left, the guy behind the 22 INTERNATIONAL PROJECTIONIST February 1950