still take place but spatial distortions are immediately seen. L15] This situation is 
unnatural for most people, but some have ocular defects such as acute astigmatism 
which produce distressing visual distortions, relieved only by wearing special 
spectacles. 
A difference in the illumination between the two eyes, even in the ratio 1000 :1, 
has very little effect on stereoscopic depth discrimination, provided that the targets 
move only in depth. t7] However, when the targets move also at right angles to the 
line of sight, in the X T-plane of photogrammetry, different illuminations may pro 
duce the Pulfrich phenomenon, which, in fact, was discovered with a photogram- 
metric plotter. [4] If, for example, the view for the right eye is brighter than for 
the left, a target moving towards the right will appear closer than its true position, 
and so on. If the illuminations are the same, but the colours are different, a less 
pronounced but similar effect occurs, which fortunately does not seem to trouble 
photogrammetric users of anaglyphs. A crude explanation of the Pulfrich pheno 
menon is that different colours or different illuminations in the eyes stimulate the 
visual cortex after various time delays, which for a moving target are interpreted as 
due to differences in depth. However, much work concerning many branches of 
visual science remains to be done on this subject. 
This last example is given not as a new discovery, but to show how 
photogrammetry has stimulated visual science in fields extending beyond stereoscopy 
to colour vision and to the nature of the neural connexions between retina and 
cortex. Science often progresses by such a dialectic between basic discoveries and 
their practical applications. The nuclei of fresh developments are found often by 
chance, sometimes by inspiration, and can also be stimulated deliberately by arrang 
ing discussions between people with different but related interests. This, of course, 
is one important purpose of the 1964 Photogrammetric Congress. 
This paper is published with the permission of the Director of the National 
Physical Laboratory. 
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