Motography (Apr-Dec 1911)

October, 1911. MOTOGRAPHY 177 lens is generally combined with a double convex second lens, and this form of condenser is generally known as the "meniscus." A meniscus lens is one having" one of its surfaces concave, and the other convex to such an extent as to make the center of the lens much thicker than the edge. Now, in such a combination the condition of least spherical aberration is not when the light and screen are at equal distances from the condenser, but is best fulfilled when the light is nearer the concave surface, and the screen is farther than the position of equal conjugate foci. Of course, the light and screen are reversible for position, as previously explained. I am now speaking" of our experimental testing screen, and not of the screen which receives the animated picture. The meniscus condenser has its lenses so placed that the surfaces are in the following order : Next the light the concave surface of the first lens, then its convex surface ; the second lens is nearly always symmetrical in shape, so it is immaterial which of its surfaces is next the convex surface of the first lens. Condensers have been made of so many different forms and with such various combinations of lenses that it is quite impossible even to enumerate them in the space at my disposal. For practical purposes this does not matter, as in the projector only the two forms I have already indicated are in general use. There is another discrepancy in the action of the condenser, called "chromatic aberration." This is common to all single lenses, and also to all combinantions of lenses, if made of the same quality of glass. This fault can easily be seen when the disc is thrown on the gate of the machine. The outer edge of the disc is of a red or claret color, and consists of red rays which are not so much bent out of their course as are the other or more refrangible rays. This is too complicated a subject to treat here, and is not of great importance to the operator. All he has to do in the case is to be sure that the disc thrown on to the gate is large enough to cause these colored rays to fall outside the film opening, otherwise they will appear on the picture screen, generally in the form of red or brown corners or edges. It is very difficult to lay down rigid rules as to which style of condenser is best to be used in conjunction with any special forms of projecting lens. In the first place, the diameter of the projecting lens has its bearing" on the subject ; and in the second, the distance of the condenser from the projecting lens also has its effect. By altering this distance it is often possible to find a place at which a somewhat unsuitable condenser will produce fairly even illumination, but this is usually attended with some loss of light. Generally speaking, a double piano works well with any projecting" lens up to 4 inch focus, and a meniscus from and including 4 inches and upwards. Theoretically, there is only one focus and one form of condenser exactly suitable for one focus of projecting" lens when used at one definite distance from the screen, but the discrepancy in the condenser comes somewhat as an advantage because owing to its rough and ready optical properties its possibilities— never exact enough to get its theoretical best — render it flexible in application. If we could have condensers without discrepancies, and absolute points of light of sufficient power for our illuminants, quite small projecting" lenses would pass all the light possible, and our illumination on the screen would be double that we now eet using the same amount of light, and the heat woujd be much reduced in proportion to the effect obtained. The condenser, as we now use it, sends rays from many directions on to the film, and some of these rays not being in the required direction, strike the tube of the projection lens after passing through the film; they consequently take no part in forming the picture but they carry unnecessary heat to the film. This is another disadvantage due to spherical aberration ; and chromatic aberration is responsible for loss of some light in that the outside of the cone of rays must be carefully kept from being allowed to pass through the film. Much improvement is possible, and it is rather curious in this age of advance that nobody has so far seriously taken in hand the development of condensers for use in projecting machines for animated pictures. THE PROJECTING LENS. The projecting lens almost always consists of a combination of four lenses mounted in a tube. On taking such a combination to pieces for the purpose of cleaning, only three lenses will be at first seen, but closer examination will show that one of the lenses is composed of two glasses cemented together. This lense does most, if not all, of the work of the combination, and in it the chromatic correction takes place ; while the two other lenses (one negative and one positive) correct the spherical aberration and the distortion of the image which usually results, when a combination such as the cemented pair is used by itself. The two lenses also often help to some extent to produce the image. In some makes, about half of the magnifying" power lies in these lenses ; in others, little if any power will be noticed if they are looked through ; but in any case, an object viewed through them will be much distorted towards the edges of the field. This distortion is of the opposite kind to that seen looking through the cemented lens ; the two distortions acting in opposite to one another ought to produce a true image on the screen. It is very important that the lenses should be placed in their right order in the tube mount, also that each lens should have its proper surface facing the screen. Between the two separate lenses will be found a ring of brass. Special care must be taken of this rin°", because its thickness limits the amount of separation between the lenses, and on this exact amount of separation the excellence of the action of the whole combination depends. In putting the combination together, first see that the cemented lens has its curved surface towards the screen, next that the deepest curve of one of the separated lenses is placed in the hollow curve of the other, with the brass ring between them-; again, the deepest curve must be "towards the screen. "All 'the most pronounced curves face the screen," is a good and easily remembered rule by which to work when handling this very confusing combination of lenses. I have seen many performances spoiled by inattention to the correct placing of these lenses after cleaning. The corrections in the projecting-lens are the result of mathematical calculations of a very complicated order, and the outcome of many years of work ; nevertheless, so difficult is the problem that it is not yet completely solved. Broadly speaking, the shorter "the focus, the larger the aperture ; and the shorter ,the mounting tube, the more difficult it is to get mst-class definition without distortion of the image. By the use of longer tubes, better definition and less distortion can