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plurality of thin glass plates, spaced apart, served the purpose admirably. The only trouble encountered was the breaking of the plates through which the light first passed. This, because the plates were heated in the region of the light spot faster than the glass could conduct it to the parts of the plate lying outside the path of the light.
To overcome this difficulty, a water cell was made into which glass plates were put, the water serving the purpose of equalizing the heat over the whole surface of each plate. No more breakage was then encountered. In order to dispell any doubt in your mind about the true reason for the results, I might add that after the cell was filled with water, but before the spaced glass plates were put in, the film ignited in from 32 to 35 seconds, but that after eight or ten plates were put in, the film was not ignited, or even puckered, after repeated tests of 25 to 30 minutes duration. This device cuts ofiF some light — I have not yet had time to measure the loss — but a 50 per cent loss is permissible and still have as much screen illumination as results from the running of the machine without it but with the usual 50-50 shutter.
In this connection it may not be amiss to caution any of you who may wish to construct such a cell, that the cell must be not less than about three times the width of the beam of light. I am not yet ready to say just why a narrow cell is not as good as a v/ide one in preventing the ignition of the film, and so will reserve a positive statement until from repeated experiments I can be certain of it.
Another thing which is interesting is that the same number of plates in surface contact with each other will not prevent ignition. A mica cell with spaced sheets is also effective, but the color of the mica and the flimsy, thin sheets are objectionable.
Another observed phenomena which gives further evidence of the complexity of the projected light is that: —
There is a point between the condensers and the aperture plate at which a pencil, screw driver or other slender object held in the cone of light does not cast a shadow on the screen.
This seems incredible, but comes about probably because the heat rays, the light rays, and the color rays, which had, therefore, been more or less separated, so criss-cross and mix-up at this point that there are not enough directed rays to cany a defined image to the screen.
Further investigation of the light in this region is being made and will perhaps be presented in another paper following this primer presentation of the subject of condensers in motion picture projecting machines.
In concluding, let me encourage you to original research, for the field is full of opportunity for profitable investigation. Stop a moment and think of it; no material change has been made in the mechanism, and no change whatever in the principle involved in projecting machines since the first projecting machine was deposited in the U. S. National Museum twenty odd years ago. It had the