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would even then not be able to get away from the diaphraming action of the aperture plate.
Dr. Kellner : If you extended your source
Dr. Story: If, with any condenser sufficiently large to cover the entire aperture, as seen from every point of the projection lens when these three units are in position you use a large enough source, you will then have as much light as you can ever get through that projection lens wnth a source of that intrinsic brilliancy, barring reflection and absorption losses. Of course, since the ordinary source is square, and the aperture plate is wader than it is high, the image must lap over the top and bottom of the aperture a little bit even with a perfectly corrected condenser, in order to get as jiiuch breadth.
Mr. Victor : I would like to ask Dr. Story a question. These tests, w^ere they made under practical projection conditions? By that I mean, was all the flux passing through useful ? Another thing you mentioned, that if the small condensers were used wath a half inch source of illumination, the result would undoubtedly be superior, but that we unfortunately are unable to get such a large source in such a small shell as the small lamps would permit. But it is my impression that the v^O volt 165 watt lamp which I submitted to you did have practically a half inch source. I may be mistaken about that, but I w^otild like to find out if I am right or w^'ong.
Dr. Story : That source, with its image in the spherical mirror, was 9/32 of an inch wide by 5/16 of an inch high. I w'ould like to repeat one thing about the tests. No attempt was made to have a uniform screen. We were simply considering maximum possible illumination. Of course, you cannot use the 4.5 inch condenser focusing a source on the aperture plate, unless that soinxe is a uniform source, but we had found before that the prismatic condenser gives more light than the plano-convex at its best ; the prismatic giving you also a uniform ilkmiination. In other words, the plano-convex w^as here used merely to tie up this experiment with the previous one in quantities, leaving out entirely the effect of imaging on the screen, w^hich, of course, you are bound to get with the vise of any incandescent filament focused on the aperture plate. Of course, if you are going to avoid with this particular large condenser, that image, you must focus then in your projecting lens or near it, and you are bound to get a very decided diaphragming by the aperture plate. But that is only in the case where yovi take something else into consideration than the total quantity of light in the condenser.
Mr. S. C: Rogers : I would like to say in regard to focusing the filament on the aperture plate, that in all the experiments and tests that we have made, both with the 4j^'' diameter prismatic and planoconvex condensers, meniscus combinations and reflector combinations, we have found that the maximum light w^as never obtained when the filament was focused upon the aperture plate. This does not necessarily mean that the screen is uniform or usable, it means only that the image is focused somew^here else either inside or outside the objective lens, depending upon the focal length of the lens.
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