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in projection without an un-uniformity of illumination on the screen, the dummy objective was replaced by a real objective lens, and the image of the aperture thrown on the screen. The objectives at hand were 2f^-inch effective diameter, 5^ -inch equivalent focus, and 1^-inch diameter, 4-inch focus. The latter is of about the same speed as a 1 34 -inch diameter, 5^ -inch focus, and will give approximately the same total illumination curves: It was because this 4-inch equivalent focus lens was the only small diameter lens at hand that the second dummy objective used in the curve was 1^ inches in diameter, instead of 1^ inches as previously.
With opal glass .3 inch and .5 inch square, and a 900-watt incandescent filament lamp as sources, with each of these objectives and with each of the three condensers, and each of the d-^ used in Figs. 13 to 17, d2iW3iS varied and the uniformity of illumination of the screen carefully observed. For each arrangement, the range of values of d., for which the screen showed the following characteristics was noted :
ILLUMINATION NOTATION
1. Uniformity
2. Dark center o o o o o
3. Dark corners
4. Black corners — — — — —
5. Streaks I I I I I
6. Image of the Source II II II II II
Of these the last two were noticeable only when using the incandescent filament. Of course, these observations involving as they do merely the appearance of uniformity, though not being at all accurate — as shown by the many inconsistencies of the readings of do — are nevertheless exactly the kind of observations that affect practical projection.
Tracings were next made from the plates, of representative curves of each source size, condenser and objective combination and the different parts of each curve drawn in the appropriate symbol or symbols, where several kinds of un-uniformity overlapped. Tracings for the 900-watt incandescent filament lamp were made, of course, from the plates taken with a .5/10-inch source, the symbols being determined by observation of the screen as described above. Some of these tracings are shown in Figs. 18-24. The objectives are called 2^-inch and 1%-inch in these figures, but they were really, as has been pointed out, 2^-inch diameter, 5^inch focus and 1^-inch diameter, 4-inch focus.
Figs. 18 and 19 show the distribution for different values of ^/-L when the piano convex and prismatic No, 3 condensers are used with a .3-inch source. The subscript ''a" refers to the 2^-inch objective and "b" to the 1^-inch objective. Figs. 20 and 21 give the same Condensers with a .5" source. These curves show the gain in imiformity as well as brightness when the larger source is used. Figs. 22 an'cl 23 taken in connection with the last plates show the effect of the coils of the incandescent filament on screen uniformitv with the different condensing systems. Fig. 24 shows the same thing using prismatic No. 2.
It is apparent from Figs. 18 to 21 that with sources of uniform
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