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by the size, the weight and the complexity of the equipment which he can carry.
Now, we have learned, . if I am not mistaken, something this afternoon that we were not aware of before, and that is that the small condenser system is capable of giving about 50 per cent more efficiency from a definite light source than the large condenser system. On the other hand, we are told that the limitations of the small condenser system are those of extended size of source, again correlating with the necessary diameter of the bulb which can carry that source without destruction. It seems to me that in every instance the engineer has the problem before him of deciding the proper relation between the size and amount of his light source, the size of his condenser, the size of his projection lens, and the size and position of his shutter. It does not do any good to get more light through a larger projectin lens, if your spot from that projection lens on your shutter is so large that you have got to sacrifice a considerable proportion of it by an increase in the width of the travel blade of your shutter. It is entirely a matter of practical compromise, just as it is in almost any other line of human endeavor. You may shorten the life of your lamp and increase its efficiency or vice versa, but there is no ideal system of the optical train which is adaptable to all the conditions that arise. There must necessarily be an appropriate design of the instrument for the specific purpose to accomplished.
Dr. Story : Of course no projection lens is perfectly corrected over its entire area. A particular zone — a latitude, so to speak — is calculated to give minimum aberation, and the zones inside and outside of this will be less perfectly corrected. The larger the diameter of the lens, other things being equal, the greater will be these imperfections. If the condenser delivers most of its light to the outer zone of a lens having its corrected zone near the center, it is obvious that the image on the screen will not be as sharp as if the majority of the light had passed thru the more perfectly corrected middle zone. Another lens may be corrected for minimum aberation in an outer zone, so that the light delivered to the center does not produce as sharp an image as that thru the edges. Since an appreciable quantity of light cannot pass thru an infinitely narrow zone, there will always be a compromise between illumination and sharpness. Since the area of narrow zones of equal width is proportional to their diameters, if the condenser and source are large enough to fill the whole lens, we get better definition if the lens is corrected for an outer zone rather than for the central portion, since the greater part of the light passes thru these outer regions.
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