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differences of this size are by no means insignificant when counting the cost as recorded at the meter. Illumination tests were made by substituting the test plate of a portable photometer for the camera objective used in taking the photographs illustrating this article. In making such photometric tests the actual screen illumination obtained with a given system depends upon so many elements, such as the equivalent focus of the objective, magnification, screen distance, density of film exhibited, exact shutter design, individual lamp used, age and condition of lamp, reflector, and line voltage at the time of making measurements, that it is hardly within the scope of the present paper to give detailed measurements of actual screen illuminations, but there are recorded comparative measurements of each system in Figs. 10-12 and in table i.
Table I
SCREEN ILLUMINATION WITH DIFFERENT CONDENSING SYSTEMS FOR LARGE AND SMALL DIAMETER OBJECTIVES
Separation Two Plano-convex Coming Corrugated
between Condenser Lenses Condenser 4 j^"
Condenser and Ayg" diameter diameter
Aperture Plate 73^" focus 2^2" x 63^" foci
Center Center Edge Center Edge
Large Objective 63^" Streaked Streaked 32,4oo 2 3,4oo
sS'Fo": •.::::::/ '^'^^^ "'^oo .7,200 16.100
Small Objective 63^" Streaked Streaked i4,8oo 13,900
5}|'^~;.::::::) ^^-^^^ 9,300 ..,65o 8.4oo
Examination of Table i shows us that if the plano-convex condenser system is used with a 63^'' separation it is impossible to get an evenly illuminated field with either objective. With a 12'' separation a fairly even field may be obtained, free from streaks, and the intensity at the edge of the picture is at least 70% of that at its center. The intensity is substantially the same as with the corrugated condenser for the same separation. To definitely determine whether with this separation there is any consistent difference between the plano-convex and the corrugated systems would require a much more extended series of measurements than was possible to make at the present time. The table does, however, show definitely two things.
First, with every condenser system a much higher screen illumination is obtained with the large than with the small diameter objective.
Second, with a given objective a much higher illumination results with the corrugated condenser at 6j^'' separation than can be obtained with either condenser at 12" separation. The fact that owing to its design the corrugated condenser gives an even screen illumination at 6}/^" separation, whereas the plano-convex condenser gives a decidedly uneven or streaked illumination with a separation substantially less than 12'', points to the conclusion that for highest illumination efficiency the corrugated condenser used at 63^'' separation is the most desirable condenser to use in connection with the Mazda projection lamp.
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