The international photographer (Jan-Dec 1939)

r'lected light. It is the action of the various chemicals upon the exposed dts contained in this thin layer of emulsion with which photographic cmistry has to deal. DEVELOPING: Upon exposure in a camera, the grains of the silver ]f>mide, chloride or iodide, depending upon which is used, to form the lht-sensitive material of the emulsion, are so effected by the light falling ( them as to form the invisible latent image. Certain reducing agents ileveloping agents* known as REDUCERS have the power of removing the llogen (bromine, chlorine and iodine) elements from the silver salts, that Te acted upon by light, and leave a deposit of black metallic silver which 1ms the visible image. This occurs because the developing agent has a iich stronger affinity for the exposed halogens than the metal of the silver ! t itself. These halogens, thus extracted from the exposed silver bromide, ddize the developing agent and it is the bi-product of this oxidation that Sickens the exposed silver. These reducing agents, besides having a M>ng affinity for the exposed halogens, are also rapidly oxidized by oxygen [>m air in the solution. To prevent this further oxidation, another comjund known as a PRESERVATIVE is added to the solution. This chemik is itself oxidized instead of the reducing agent. It was early found that the process of development takes place very 2) >wly or not at all unless the solution is alkaline. For this reason an ■cali is added to the solution to speed up development. This agent is iown as the ACCELERATOR and swells or softens the gelatine, periling greater access of the solution to the emulsion. However, a deloping solution, containing only these three agents, works so energetically to also react upon the unexposed silver salts, thereby causing a chemical 2 on the film. To prevent this occurrence, a fourth agent known as the ^STRAINER is added. The function of this agent is to slow down the tion of the ACCELERATOR, thereby balancing the developing solution that it acts efficiently and with the required amount of energy upon e latent image. DEVELOPING AGENTS Agent Remarks Removes bromine from the exposed silver-bromide grains, leaving black metall REDUCER PRESERVATIVE ACCELERATOR RESTRAINER ic silver. Prevents excessive oxidation of the reducer and preserves the solution. An alkali which softens and swells the gelatine emulsion and speeds up the rate of development. Slows down action of accelerator so as to prevent reduction of unexposed silver bromide and fogging of material. The above four agents, when dissolved in water according to the formus that have been worked out to produce the best results, form the vcloping solution. The exposed negative or print, after being immersed this solution for a suitable length of time in the dark, or under a suit able light (safelight) is developed causing the latent image to be transformed into a visible black silver image. FIXATION: The unexposed silver bromide grains are not effected by the developing solution, therefore they still remain in the emulsion after the visible silver image is formed. This unexposed silver bromide is still sensitive to light and if not removed would darken upon subsequent exposure. Sodium Thiosulphate (Hypo) is capable of dissolving this silver bromide from the emulsion. The next operation, then, after developing, is to immerse the print or negative in the "fixing bath". In this bath, as in the developing solution, there are several different agents. FIXING AGENT ( hypo I dissolves the unexposed silver bromide from the developed negative, leaving a clear negative image of the subject. Since a certain amount of alkaline developing solution is carried over into the fixing bath with the negative, and since development will continue as long as this alkalinity exists, a small amount of acid is added to the fixing bath to NEUTRALIZE the alkali and stop development immediately. The reducing agent of the developing solution is subject to oxidation upon being carried over into the fixing bath, which tends to cause stains on the emulsion, and also in an acid solution the fixing agent tends to decompose, liberating sulphur. To prevent this oxidation and decomposition, a PRESERVATIVE is added to the bath. A fourth agent known as the HARDENER is added (33) to practically all fixing baths. Its action is to harden and prevent further or undue swelling of the gelatine emulsion during subsequent washing. It should be understood that the action of the hardener, although highly desirable for mechanical reasons, is not essential in fixing the image. AGENTS OF THE FIXING BATH Agent Remarks Dissolves undeveloped silver bromide from FIXING AGENT emulsion. Neutralizes alkali carried over into fixing bath NEUTRALIZER by prints or negatives. Prevents oxidation of reducing agent and dePRESERVATIVE composition of fixing agent. Hardens gelatine and prevents further swellHARDENER ing. (This action is not essential to actual fixation.) WASHING: After fixation there remains in the emulsion, in addition to the insoluble silver image, a number of soluble compounds resulting from the reactions of developing and fixation. If these were allowed to remain in the emulsion, the image would fade and become discolored in a short time. Therefore, after fixation, the negative or print is washed in clean water for a period long enough to remove these soluble compounds and leave in the gelatine nothing but the silver image. It is then ready for drying. (To Be Concluded in August) • •• )ROJECTION SYMPOSIUM, pARTS, VII., VIII. art Two of Academy Research Council report on thorough and technically accurate planning of test reels for stanjrd theatre sound program; standard and push pull sound track usage in present production from major lots. ART VII: Final installment of Academy report on test reels. By JOHN HILLIARD MGM Sound Department For the purpose of determining the acoustic ?ponse of the horn systems and of the auditorm we have made up Standard Warble-Tone ■st Reels. Secondary Standard prints are available, in ith Variable Area and Variable Density, each ntaining approximately the same frequencies as e included in the Multi-Frequency Reels. Each frequency in the Warble Tone Test Reels is a warble of ± 5 per cent on all frequencies, is degree of warble having been chosen so that anding waves will be minimized in the audirium. Through the use of a microphone in conjunc>n with an amplifier system and a sound level eter, the acoustic response of the sound system id auditorium at the various frequencies can be ■termined. Under normal conditions at least five different microphone positions in the auditorium are used, and the readings are averaged to give the acoustic curve for the auditorium. To determine the acoustic response of the speakers, the conventional method of measurement involves the averaging of 5 or more readings made with the microphone close to the speakers. However, in making these measurements care must be taken to select microphone positions which will not favor the response of either the high or the low frequency units. These warble tone prints are calibrated exactly as are the Multi-Frequency Test Reels, that is, against the same calibrating reel and on the same equipment set-up. To check the lateral alignment of the scanning slit we have a Standard Buzz Track I Figure 5). The opaque track is 86 mils wide. On the picture side of the track there is a 300 cycle tone and on the sprocket side a 1000 cycle tone. These tracks are so spaced that if the scanning slit is properly placed and of the correct dimension, no tone will be heard from the reproducer, but if the scanning slit is improperly placed toward the picture side the 300 cycle tone will be heard, and if misplaced toward the sprocket side the 1000 cycle tone will be heard. A loop prepared from this track is run in the equipment and the scanning slit laterally adjusted until no tone is heard. In making up these prints we hold the track placement to within ± 2 mils of the correct position. This track thus provides a means of adjustment of the position of the scanning slit to the current positioning tolerances. After the scanning slit has been checked for proper dimension and placement, it is of course necessary to check the uniformity of illumination across the scanning slit, and for this purpose we have made available a Standard Scanning Illumination Test Track, which contains 17 approximately equally placed 1000 cycle tracks, each with an amplitude of 6.8 mils ± 1.6 percent. ( See Figure 6) . If the illumination on each track is constant, the output as measured with a VI meter will be constant, but if the illumination varies the amount of this variation may be read directly on the VI meter measuring the output. Of the 17 different tracks, the outside two and ■JTERNATIONAL PHOTOGRAPHER for Jul). 1939