reported and analyzed some of the experiments conducted by the University 
of Rochester Institute of Optics under the sponsorship of the U. S. Army 
Engineer Research and Development Laboratories. In the phase of the 
experiment concerned with unbalanced illumination, 30 observers made 
measurements on a stationary target appearing in a stereo model. With 
a 50 per cent difference in illumination between the pairs, a 2.4 per cent 
loss in the ability to repeat vertical stereo measurements was found. 
It was decided that quantitative determinations on a moving floating mark, 
such as occurs when contouring a stereo model, instead of on a stationary 
target as used in the Rochester experiment, would provide valuable data. 
The phenomenon that quick lateral movements cause peculiar disturbances 
in establishing the coincidence between the floating mark and the observed 
object when viewing stereoscopically was recognized in the early develop- 
ments of photogrammetric instruments by Dr. Carl Pulfrich. This effect, 
first suggested by F. Fertsch, bears the name of the Pulfrich Serene / 
phenomenon, and was first described and analyzed by Pulfrich in 1922. 
It is called the "Circling Marks" effect by Von Gruber. 
These disturbances were first evident to Pulfrich when the pencil of 
the instrument traced a different curve on following the same contour with 
the directions reversed. At first the cause was attributed to the density 
of the photographic plates, but finally the cause was determined to be the 
difference in plate illumination. The explanation is based on the hypothesis 
that there is a visual latent period whose magnitude is inversely related 
to the intensity of illumination. Thus, if one photograph is brighter than 
the other and the spatial mark is moved toward the brighter, the eye grasps 
the motion of the mark on the duller one less readily than the brighter, 
and therefore imagines that it still sees the mark on the duller photograph. 
Consequently, the floating mark appears to sink into the ground when the 
motion in the direction of the dull side is stopped. 
Insofar as is known, no quantitative data have been published concerning 
the magnitude of this effect in stereoplotter operations although data have 
been published for other types of observations with unbalanced illumination. 
In this limited experiment conducted at the U. S. Army Engineer Research and 
Development Laboratories a stereo model of six-inch focal length photography 
exposed at 10,000 foot altitude was oriented in a Zeiss C-8 Stereoplanigraph 
at 1:13,000 model scale. Density of the plates in the area of the experiment 
was essentially the same. A Weston Photometer was used to measure the 
illumination falling upon each diapositive. The X-motion handwheel was 
turned at a constant speed by the supervisor and the distance traveled was 
timed by an electronic timer. The observers kept the floating mark in 
  
3/Carl Pulfrich, Die Stereoskopie im Dienste der isochromen and heterochromen 
Photometrie, Naturwissemschaften, 10, 1922. 
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