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as long as the condenser exhibits no yellow color. A caution must be voiced, however, along this line, for color, or lack' of color,, in glass may be due to absorption in two or more regions' of the spectrum, and so may lead to error in the comparison unlpss an optical check is made in each case. With the condensers used in the present tests, the readings were considered close enough to justify the adoption of those given by the cell as representing the effect on the eye, with a slight reservation in favor of prismatic No. 2.
Fig, 12
COMPARISON OF PHOTO ELECTRIC CELL CURVES WITH PHOTOMETER READINGS.
Light with Cond.
Source
Photometer
Theoretical Limit
Side
Volts
di
d2
Amp. Readings
Av.
Photometer Cell
Prismatic Condenser No. £
j
125
25^
.35
1.89 1.96 1.90 1.94
2.0
1.93
.55
192 1.90 1.96 1.90 Theoretical Limit
1.95
2"
125
.35
3.43 3.75 3.50 3.80 3.55 3.35 3.50 3.20 Prismatic Condenser No. S
3.40 3.60
j
3.51
^"
110
2^
5^
,35
1.45 1.15 1.14 1.10
1.19
1.18
.52 .51
Theoretical Limit
2"
110
.35
2.35 2.25 2.15 2.15 2.26 2.45 2.35 2.15 Meniscus Condenser
2.30
%"
125
5
8V4
.35
2.15 2.10 2.20 2.05
2.15
2.15
.602
2.05 2.15 2.30 2.20 Theoretical Limit
2.15
2"
125
.35
3.45 3.72 3.50 3.70 3.10 3.80 3 85 3.60 Meniscus Condenser
3.65 3.30
3.57
V2"
110
5
m
.35
1.18 1.30 1.22 1.34
1.24
1.33
.584 .58
l.,32 1.26 1.34 1.27 Theoretical Limit
1.23
2"
110
.35
2.30 2.25 2.35 2.35 2.15 2.35 2.30 2.20 Piano Convex Condenser
2.20 2.35
2.28
¥2"
125
5/8
m
.36
1.79 1.72 1.84 L79
1.76
1.79
.594 .59
1.90 1.77 1.77 1.78 Theoretical Limit
1.81
2"
125
.36
2.95 3.15 2.8 3.0 3.15 3.0 3,1 2.95 Piano Convex Condenser
2.8 3.2
3.01
110
5/8
m
.36
1.08 1.05 1.07 1.15
1.06
1.07 1.05 1.05 1.07 Theoretical Limit
1.05
1.07
.594 .59
2"
110
.36
1.78 1.91 1.80 1.72 1.86 1.80 1.80 1.75
1.84 1.80
1.81
Motion-picture projection does not concern itself with the total quantity of light alone. Uniformity of screen illumination is also a factor. Five plates were made as a basis of curves showing uniformity. Fig. 13 shows two families of curves made with a piano convex condenser, one with a 2^-inch and the other with a 1^inch dummy objective. Each family consists of curves differing in as before. The maximum ordinate of the upper family has been assumed as .59 from Fig. 6 and the others calculated from their proportional lengths. Figs 14 and 15 are the same kind of curves made with prismatic No. 2 and prismatic No. 3, although, of course, with different actual values of d^. These three plates were made with an opal glass source .5-inch on a side which is approximately equivalent in area and uniformity to the crater of a direct-current arc of 110 amperes, and in area to the 900watt incandescent filament lamp. Figs. 16 and 17 were made with piano convex and prismatic No. 3 condenser respectively, and .3-inch source, equivalent to a 33-ampere arc.
To determine what part of each of the curves could be used
110