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But at best the heat is such that the condenser gets very hot though the heat of itself alone does not crack lenses. In actual practice I have found that if the arc lens is held in a non-conductive ring it will never break no matter how hot it gets. Lenses crack because of unequal stress in the glass, and this comes about because something has carried away heat from a limited area and not from the whole mass evenly. Thus, if one should touch a very hot lens with a piece of metal, say, a screw driver, the lens will crack, because the metal being a better conductor of heat than the glass robs the glass of its heat at the point of contact and the equilibrium of stress is disturbed and the glass cracks. I have had the glass crack across between my thumb and finger when I attempted to pick up a hot lens by its edge (without knowing before that it was hot). If the lens is heated evenly and remains so, that is, is not robbed of any part of its heat by a conducting or convecting medium it will never break. A complete understanding of this phenomena and its proper recognition by the operator, would enable him to get a much more brilliant screen picture with much less current consumption.
It is exactly the same with many other improvements which might be attempted if we had graduate engineer operators to handle our machines. So the best we can do is to make a compromise machine and wait until the public grows up to our ideals. In this category is an adjustible shutter; and the three-lens condenser system IVe just been talking about; and, to come closer home, a multiplenegative carbon arc with its single unshadowed crater in the exact axis of the optical system, an ideal arrangement which we had to give up because we couldn't take the time to teach each operator where such a lamp was installed, if, indeed, he were teachable.
Much of what is here explained could be calculated with exactness and applied by rule if the source of light were an infinitely small point, and condensers were as carefully ground and annealed as projection lenses, which, however, isn't the case. Also the condensers and light source area have a definite bearing on the sharpness and brilliance of the screen picture aside from the question of illumination. The optical system of motion picture machines is a makeshift, and I hope that some of us will undertake a systematic investigation of it looking to a more definite knowledge and resultant improvement.
In conducting experiment with motion picture machines it is desirable, on occasions, to take the heat out of the light so that the light may shine on the film continuously for an indefinite time without igniting or puckering the film. The usual plan is a tank of water, located between the condensers and the picture aperture, through which the light must pass. This is a help, perhaps, but not wholly effective. Alum is sometimes added to the water, but, so far as my experiments go, adds nothing to its heat absorbing property.
After repeated trials and finding that the water cell did not furnish the required protection, I set about to find out what would, and developed some surprises. Our first surprise was the discovery that ice water was less effective than warm water. Next I found that a