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case be multiplied by the efBciency of the projecticn lens used — that is by the prcportion of the light it brings to a focus, to that falling upon it from the aperture.
Three small condenser combinations were compared with the 4.5 inches plano-convex previously used (Ice. cit.). Their approximate diameters and fecal lengths were as follows :
Focal Diameter Length
Condenser No. 1 1^" Y^"
Condenser No. 2 \%" 1 H^"
Condenser No. 3 1%" Y,"
That di with which the greatest deflection could be obtained for each combination of source, condenser, aperture and projection lens, was determined in each case without making a plate, but by noting the maximum galvanometer deflection as d^ was changed. For each condenser-aperture combination a plate was then made, each curve of which represents a combination of source and projection lens size, made with the value of d-^ found as described. These plates are shown in Figs. 2 to 9. The curves are all numbered, and the various combinations indicated under their numbers in Tables I and II. The values of d^ are here given in sixteenth of an ineh.
As will be seen, there is but little change in d^ for any given condenser, for changes in size of source or projection lens. This means, of course, that for maximum illumination the "image" of the source produced by the condenser should be in approximately the same place — that is, in the projection lens for the small condensers, and the aperture plate for the large.
The ordinates of the curves are not now given in terms of the maximum possible illumination with a source of given intrinsic brilliancy ("). Instead, after all the curves showing the variation of illumination with change of condenser-aperture distance (d,) were made, the condenser (C), aperture (A) and projection lens (P) were removed and the source increased to 1 inch on a side. This source was placed 8 inches from the opal glass (O) of thf photoelectric cell assembly and the deflection of the galvanometer thus obtained. The cell assembly was then moved back slowly from the source, this deflection, of course, decreasing approximately as the square of the distance (d). The curve traced out in this way is designated by R in the figures. It will be noted that tne scales of d and di are the same, the d zero has been moved along 7 inches, to bring the R curve on the plate.
The opal glass of the cell assembly was 2.75 inches in diameter. Accordingly, the light in lumens falling on it from the 1 inch source was approximately
~u. 2.7521 I
L = -^— =5.95 —
4 d^ d?
where d is the distance from the source (S, Fig. 1) to the opal glass (O) and I the intrinsic brilliancy of the source. Assuming
convenient values of -y, corresponding values of d can be cal
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