Motography (Apr-Dec 1911)

176 MOTOGRAPHY Vol. VI, No. 4. reason. Using a convex lens with the paper almost touching it, the light will be only slightly condensed, but on increasing the distance between the lens and the paper, the circle of light will become smaller and more intense, till a point is reached at which an image of the sun is formed. This is called the principal focus of the lens. When a lense is described as of 4 inch focus, it means that such a lens is capable of bringing parallel rays to a point at a distance of 4 inches from the lens. With a system of lenses like that in the projecting lens, it may be difficult to find the point from which to measure. A concave lens has no definite point of focus, but its strength, or power of bending light rays, is indicated by stating in inches its "negative focus." It has the power of bending light outwards to the same extent as some other lens has of bending it towards the center. Placing two such lenses together, the effect will be about equal to a parallel plate of glass, and neither condensation nor dispersion of light will be produced. The opposite effects of the two lenses will balance one another, and the power of the concave lens will be equal to the convex lens acting against it. A concave lens, which will neutralize the effect of a convex lens of 4 inch focus, is consequently called a 4-inch negative lens. In the projector we do not meet with concave lenses except in the projection lens itself, which contains two ; but as both of these are used in conjunction with more powerful convex lenses, the phenomena attendant on the use of concave lenses need not be understood by the operator. The principal focus of a convex lens is, then, the distance from itself at which it is able to bring to a point parallel rays passing through it — the sun's rays are to all intents parallel. The manner in which a lens bends rays of light is reversible ; by which I mean parallel rays passing into our lens being brought to a point at a definite distance. Now if we «)lace a luminous point at the position of the principal focus of a lens, the rays from that point falling on the lens will pass through and become parallel on the other side. If we move this luminous point farther away beyond the distance of the principal focus, instead of the rays emerging parallel from the lens, they will be bent towards the center — condensed — more and more as the point of light is removed to a greater distance. A point of focus will be found on the side of the lens away from the light, and as the light is moved away from the lens, so the other or "conjugate" focus point will come nearer and nearer. The two points are called the conjugate foci, and are interchangeable as before. The light being in a certain position and the screen being adjusted to receive the conjugate point, the position of the light and screen may be reversed and will be found correct. There is a position to be found at which both the light and the screen are at equal distances from the lens. This is called the distance of "equal conjugate foci." On measuring this distance it will be found to be just double that of the principal focus of the lens. The principal focus of a lens may be found by setting it up with a light and screen at equal conjugate foci, and taking one-fourth of the distance between the light and the screen. These facts can easily be verified by taking one of the lenses out of the condenser, propping it upright on i table, and using a cardboard box for the screen, and a wax match as the source of light. The complete condenser could also be used, but the variations of distance would be very small and the measurements would require to be made from a position midway between the lenses. The principal focus can be measured correctly enough, using a distant point of light— a street arc lamp across the road, for instance. The best lens to use for the experiment is a reading glass of 3 or 4 inches diameter, because being symmetrical in shape (both curves of the same convexity) the measurements can be made from the edge of the lens, and will be found substantially correct. Unsymmetrical lenses, and combinations of lenses, can be treated in the same way, but the point from which to measure has to be found in a somewhat complex manner, involving considerable mathematical knowledge. The main principle of the condenser will be easily realized from the foregoing ; its particular function is to condense or intensify the light on the film. It might be supposed that any lens of somewhere near the correct focus and diameter would be suitable for the purpose. This is not necessarily the case, and to explain the reason I must again turn to theory. A lens with a spherically curved surface is not a perfect appliance because it has not the power of bringing all the rays emanating from a point to a corresponding point on the other side. As a matter of fact, the rays which pass through and near the center of the lens come to a focus somewhat farther from the lens than do those rays which pass through near the edges. Each concentric band on the lens has its own length of focus, so that our point of light is represented on the other side of the lens as a line more or less long according to the power of the lens and the arrangement of the curves or flats composing its surfaces. This inaccuracy or discrepancy in the action of a lens is called "spherical aberration." It may be considerably reduced by forming the curves suitably, and nearly corrected by a combination of several lenses. The condenser as now used suffers very considerably from this defect, and this is the direct cause of the difficulty of getting even illumination on the screen. The black spot is the center of the field ; the large circle of better illumination with its illuminated corners so often metwith, are effects of spherical aberration in the condenser, and when very pronounced are due to the use of a condenser having been placed at the wrong distance from the film mask. It so happens that the condition of least spherical aberration in a symmetrical lens or symmetrical combination — and the usual double piano condenser is a symmetrical combination — is best fulfilled when it is used with equal conjugate foci, but even then the discrepancy is considerable. Condensers have been made with curves to decrease this defect, but so far their cost prohibits their use, and they only work at their best when used at the right distance from a projection lens of suitable focus ; under such conditions the gain in illumination is sufficient to warrant the extra expense, though breakage of the glass near the light would run the cost of renewals much beyond that of present practice. The condenser is not by any means an exact appliance, and the calculations necessarv to exactly deduce its best proportions are so complicated that it is much more practical to get the best result by actual tests, after the first rough calculations have indicated its main proportions. The condenser having its first lens — that next the light — of meniscus form is more suitable for use with long focus lenses than is the symmetrically shaped double p'ano. The meniscus first