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measurements. Both these companies were good enough to accede to our request and made the desired measurements. We desire to tender our most sincere thanks to the National Lamp Works and the Westinghouse Lamp Company for their aid in this matter.
These measurements have proven that a huge loss of light may occur through failure to properly design projection lenses, particularly those of long focal length, with maximum diameter and short hack focus. Lens manufacturers in our opinion, have heretofore not given this hugely important matter anything like sufficient attention. In fact, aside from increase of diameter nothing at all, so far as we know, has been done by the leading lens manufacturers to design lenses to fit the obvious need, though one lens made by an obscure manufacturer has pointed the way, and has amply proven its worth. Increase of lens diameter is not sufficient to meet the situation except in lenses of short or medium focal length. Lens diameter is necessarily limited, due to the fact that projection lenses must work in conjimction with a revolving shutter. This is particularly true where a light source of great brilliancy is used.
In our opinion there is urgent need for a re-designing of projection lenses, particularly those of long focal length. Personally we believe this can only be taken care of by designing lenses which shall in no case have a back focus greater than perhaps two inches, though the exact figure we would not care to state positively without further study on that particular matter. If this can only be done by means of a three-combination lens, then we believe a three-combination lens is the thing needed.
The following results were arrived at by the National Lamp Company laboratories after an exhaustive series of measurements of the projector light beam between the aperture and the projection lens. The same measurements by the Westinghouse laboratories gave essentially identical results. We may therefore confidently say they are correct.
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The following is a summary of data supplied by the National Lamp Company.
''Object: To determine the relative intensities of various points in vanous planes across the light beam on the projection lens side of the aperture plate.
Description: The optical system was mounted as indicated in Fig. 8, upon which appears the dimensions used for the six settings. The condensers were ordinary 6.5 in. and 7.5 in. foci plano-convex, 4.5 in. in diameter. The light source consisted of a small piece of opal glass over which was mounted a metal plate with a 0.5 in. diameter opening. This opening was illuminated by a Mazda projection lamp which gave us a light source without variation in brilliancy, and one which for the purpose gave practically the same effect obtained from the crater of an electric arc. Fig. 9 shows the mounting of the Weber photometer. In Fig. 8 the radiating rows of round dots are quarter
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