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462 History of Three-Color Photography
of the distance at which the object is viewed, can be fairly well differentiated.
The author pointed out18 that ScherTer's arguments were based on the use of black and white screens, and that the same might not apply to colored units. The following experiments were made: cinematograph film was slit lengthwise and stained red, green and blue, so that the absorptions were approximately the same as in screen-plates. Strips of the dyed film were stuck on gelatinized glass to form a linear screen, and this placed against a window covered with tissue paper. The plate was first placed with the lines running horizontally, then so that they were at an angle of 45 degrees. Seven observers and the author then determined quite independently at what distance the lines became invisible, with the following results:
server
=5
45°
M
Ba.
38,400
30,480
25,300
O*
20,420
15,545
16,460
F.
38,400
26,820
26,820
K.
24,690
27,430
16,150
Bu.
34,440
34,440
19,810
Y.
38,400
27,430
27,430
M.
33,530
25,910
21,030
W.
38,400
30,840
19,810
♦Suffered from red blindness.
The second column gives the distance in millimeters at which the horizontal lines became indistinguishable; the third column the distances for the lines at 45 degrees, and the fourth for a mosaic screen. This was made as follows : strips of the same dyed film were cut into small rectangles 13x17.4 mm. These were placed alternately in rows with their longer axes vertical, and so that under every red there wras a blue, and so on, the result being that rectangles of the same color were at an angle of approximately 37 degrees.
SchefTer has shown that the resolving power of the eye is of such an order that it will see lines as separate, if their "period" (twice their separating distance for lines equal in width to the spaces) is one-thousandth of their distance from the eye. From the above it is clear that the units of the above linear screen should have become invisible at 52,000 mm. when the lines were horizontal and at an angle of 45 degrees. In the case of the mosaic screen a rectangle was taken that would include four units and the diagonal measured, this was 41 mm. ; therefore the units of this screen should have become invisible at 123,000 mm. The following table shows the difference between theory and practice :
= 45° M
Theory 52,000 52,200 123,000
Practical mean 30,834 27,692 21,000
It is apparent from this that one can not apply the arguments of a black and white screen to colors. It very soon became apparent in the