The Moving Picture World (October 1907)

520 THE MOVING PICTURE WORLD. tone value of the subject. Between the clear glass of the highest lights and the opacity of the darkest shadows there must be a just proportion of tones and half-tones, and the longer the scale the more perfect the slide. The average slide has been reduced to two steps and seldom renders more than four steps of the scale. The outlines of the subjects are rendered in opaque deposits of silver and all else is clear glass. Such slides, though photo- graphic, are little better than reproductions of pen-and- ink drawings. First Get It in the Negative. The cause of this degradation of tonal quality is gen- erally in the making of the negative—insufficient ex- posure and unsuitable development. The exposure was not sufficient to render the shadow detail, and in pro- longing the development in a vain attempt to bring out this detail, everything becomes equally opaque. Let it be understood that a; brilliant negative will not produce a good slide; the best negative is one that may be termed thin, yet full of detail, and this can only be obtained by giving ample exposure and developing in a weak solu- tion containing little if any bromide. We are speaking here of dry plate photography, as this is in most common use to-day. More slides are also made oh gelatine plates, because they are more easily made and more easily colored than a collodion slide; but for cheapness of production in large quantities and beauty there is nothing to. compare with a gold-toned collodion slide. In future articles we will treat of this process. Developer for Slide Plates. We have been asked to give a good developer for gela- tine slide plates, and recommend the. following, which gives a pleasing warm black deposit: Ortol 96 grains Potassium metabisulphite 48 grains Sodium carbonate 1 ounce Sodium sulphite I ounce Potassium bromide .• 48 grains Water «. 48 ounces Eight ounces of this will develop a dozen plates, when it should be discarded. The stock 'solution keeps clear for months., if well corked, and may be used for nega- tives in the same proportions, of course omitting the bromide. • If. a little extra work is not a drawback, the quality of a gelatine lantern slide is always increased by carry- ing the development a little farther than is necessary and after fixation reducing with the well-known hypo- ferricyanide or any other reducer. Hints For Operators. By Maxwell H. Hite, . While the writer hereof does not claim to be the only man posted on the subject of Electricity and its proper manipulation for successful projection work, he believes that his writings can be read with profit by all who are interested in giving successful, safe and satisfactory life motion picture exhibitions, and as there are several im- portant things that go to make an entertainment of this kind a success, I shall from time to time write on the following subjects: First, The Arc Light; second, Cored Carbons; third, Wiring the Theater, and last, but not least, The Operator.; it is he who has the safety of the-, public in his hands, and right here I-wish to state that I agree with the editor of this journal, when" he says that the operator who smokes while in the operating room should be severely dealt with, as to my mind this is as foolish as the man.who went to draw a gallon of gasoline with a lighted wax candle. The Electric Arc Light. The electric arc. is a phenomenon discovered by Davj in the year 1800, when he sent a current of electricity through two carbons. After he brought the carbons to- gether he separated them slightly, that is, about one- eighth of an inch, and the current continued to pass, raising the temperature of the ends to a very high degree, while a vapor passed between the carbons. Thus, from 'this crude experiment of Davy, we learn that when a current of 10 to 25 amperes, under a pressure of no volts, is passed through, two carbons, with their ends first in contact and then gradually separated say about one-eighth of an inch, a brilliant flame, called an electric arc, is established. This arc is composed of a carbon vapor; that is, the high temperature caused by the pass- age of the electric current through the resistance at the contact surfaces causes the carbon to boil, and the vapot arising therefrom, being a better conductor than the air, carries the current across the air gap from one carbon) tip to the other. This volatilization occurs mainly at the I tip of the positive carbon, where the current enters the arc, and this is the point of the highest temperature and the greatest light-emitting point. The arc being main- tained at this point, the disintegration of carbon takes place, i.e., the carbons waste away and a cup-shaped end is formed on the positive carbon, called the crater, while the negative carbon becomes conical in shape. The nega- tive carbon being lowest in temperature the positive car- bon continually feeds it, and the deposit thus fed to the negative carbon forms a pure graphite. Both carbons are consumed, but the positive carbon is consumed about twice as fast as the negative. The light emitted by any heated substance increases with the temperature. The temperature of the crater when in the state of ebullition is about 3500 0 C, this be- ing the hottest point of the arc, and therefore it is from" here that the most light is emitted. In using a direct current hand-feed arc light for projection work such as moving picture and illustrated songs, it is necessary to place what is known as a rheostat in series, in order that the operator may be able to control the amount of cur- rent; without a rheostat an arc lamp would not work, owing to the fact that too much current would pass. In projection w6rk, cored carbons of the best quality should be used, as the presence of material whose points of vola- tilization are much lower than that of the carbon, must result in a considerable reduction of the temperature,! which will decrease the intensity of the light. Therefore, if the carbons are made from a mixture of carbonaceous powders, unless all of the ingredients are nearly of the same hardness, irregularities in both the consumption and the temperature will cause unsteadiness of the light For this reason I advise the reader to use the best car- bons that he can buy, in order that he may have a good, clear, steady.light. . > • Next in order is to so adjust your arc that the same strikes the center of the condensors, for unless you do . this you will fail to secure a bright, sharp picture, and when you fail to show a picture -that is clear and sharp, your patrons become dissatisfied and fail to return to see your show. My next article will treat the Alternating Arenas there is a vast difference between direct and alter- nating arcs.