International projectionist (Jan-Dec 1949)

chiefly from the outermost edges of the lens. Here is the important point. If the projection lens be one of those slow "gay 90's" models, it will fail to catch most of the light illuminating the edges of the film picture, and the screen image will be deprived of anything the lens is unable to "see". The result is a decided vignetting effect with a "hot spot" in the center of the screen. So we see that the more closely our lenses are matched to the optics of your lamps, the more evenly illuminated will be the picture we project. Eliminating the 'Hot Spot' It would seem feasible to overcome unavoidable, residual vignetting (due to the impracticability of using perfectly matched optics in most instances) by deliberately over-parabolizing the figure of the lamp mirror or condensing lenses. The effect would be an intensification of edge brightness of the film picture in the aperture. The vignetting effect would thus be neutralized by an opposing "error" of the lamp system, and the screen image would be smoothly illuminated. The same effect could be obtained by the interposition of a specially figured "compensating lens" behind the aperture, but the light loss in a lens made of heat-resistant glass might be considered too great to be tolerated. Action by the SMPE on this important matter would seem to be definitely indicated. The "hot spot" has devilled projectionists for all too long! The matter of light dispersion from silver grains in film emulsion appears to have little bearing on picture illumination. I cannot find an appreciable effect in the screen image which can definitely be attributed to dispersion. It is of interest to note that dispersion is absent in Technicolor and other imbibitionprocess prints. Ultra-Sensitive Photo Emulsion Requires Shipment in Dry Ice A new photographic emulsion for tracking down atomic particles is so sensitive its producers are faced with the problem of how lo prevent its premature exposure by cosmic rays during shipment to research workers. The new emulsion, developed in Kodak Laboratories, is approximately four times as "fast" as the company's existing nuclear track plates. It has successfully recorded electron tracks which are ten times longer than those previously captured photographically. Protection of the new material poses a problem for Kodak scientists. No sooner is the new emulsion made than it begins to record the bombardment of cosmic rays which strike all about us constantly from outer space. „ Intense Cosmic Ray Barrage Dr. Cyril J. Staud, director of the laboratories, said that "about six electrons from cosmic rays strike each square cm. of the emulsion every minute." This intense barrage, he pointed out, peppers the emulsion so much in three days time that, unless adequately protected, it is clouded with background streaks upon development. Packing the emulsion in dry ice for shipment may protect it, Dr. Staud indicated, because the emulsion's sensitivity is reduced How Many? Was this copy dog-eared when it came to you? How many men read it ahead of you? You would receive a clean, fresh copy if you had a personal subscription — and you wouldn't have to wait — you would be first to read it. Use coupon below. INTERNATIONAL PROJECTIONIST, 19 West 44 St., New York 18, N. Y. □ 1 year — 12 issues— $2.50 □ 2 years— 24 issues — $4.00 Foreign and Canada: Add 50c per year. Enter my subscription for Name Address City State ATLANTIC CITY'S HOTEL of DISTINCTION Devoted to the wishes of a discriminating clientele and catering to their every want and embracing all the advantages of a delightful boardwalk hotel. Spacious Colorful Lounges — Sun Tan Decks atop — Open and inclosed Solaria — Salt Water Baths in rooms — Garage on premises. Courteous atmosphere throughout. When in Atlantic City visit the FAMOUS FIESTA LOUNGE RENOWNED FOR FINE FOOD OPEN ALL YEAR Under Ownership Management Exclusive Penna. Ave. and Boardwalk at low temperatures. On arrival, it could be refrigerated and later permitted to warm up just before exposure. Dr. John Spence, in charge of research on the new emulsion, said that it "comes very close to complete recording of any nuclear particle." He said the emulsion is characterized by a uniform sensitivity of all its tightly-packed grains and a high concentration of silver halides. Commenting on its importance as a photographic tool in nuclear research, Dr. Spence explained that an atomic particle in the emulsion acts like a "skipping stone" on the surface of a pond. Procedure for Computing Data "Assuming one has a good, round disc-like stone and throws with precision," he said, "the frequency of the skips increases toward the end of its path. The faster the stone is traveling, the less the frequency of the skips in the water. This is paralleled by the passage of a high-energy particle through a nuclear emulsion. As with the skipping stone, the rate of energy loss is reflected in the number of grains along the track." From the length and curvature of the track and the grain-spacing along it, he said, information is obtained of the particle's speed, energy, and other characteristics. The emulsion can also bring new precision to autoradiography in medical research, Dr. Spence added. In autoradiograhpy, sections of tissue containing radioactive isotopes are placed in contact with the emulsion. The specimen takes its own picture when radiation from its tissue exposes the emulsion. Examination of the developed plate enables scientists to identify the location and amount of radioactive penetration in the tissue. The new material has been made so far only on an experimental basis and is not yet generally available. 34 INTERNATIONAL PROJECTIONIST March 1949