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Each institute probably has its own experience with experiments on 
Slope Exaggeration. Results are quite easily obtained, but the statis- 
tical value is often doubtful, such that publication is postponed. 
Mekel's paper came through this barrier (Mekel et al 1961), but also he 
has more data in stock, waiting for further verification (private comm. 
1961). Nevertheless he developed a small gadget for subjective slope 
determination, which allows quick and accurate slope estimations. 
[Ligterink's thesis (1964) on height and slope exaggeration is a 
photogrammetrist's approach. Where is the scientist who can give a 
simple relation between the photogrammetric parameters and this 
personal exaggeration factor? Or is a simple formula basically 
impossible? 
What is the psychological background for the fact that an uphill 
  
slope is seen steeper than a downhill slope? 
Is it due to our subjective feeling when driving a bicycle? | 
It surely has relevance to double scanning: the equal slopes of a hill 
are seen differently by student and teacher, and misunderstanding may 
result (Vermeer private comm, 1967). 
Colour, the next item to be discussed, effects occasionally our 
stereo depth perception: red seems nearer than green. This forms an 
additional difficulty when studying false colour photographs 
(Vermeer 1968). The general opinion "red is nearer, warmer, attrac- 
tive, etc." suggest a psychological reason, but a physiological 
property of the eye may well be the simpler explanation. The eye has 
to accomodate stronger for red in order to focus the image on the 
retina. In everyday life, stronger curvature of ihe eye-lens "means" 
the objects are nearer, especially so, as accomodation is the primary 
operation which partially guides other visual operations as conver- 
  
gence and pupil-size adjustment. (Fender, 1964). 
We would like to know whether training can correct the false 
impression, Vermeer's experiences are negative so far. 
COLOUR PERCEPTION is closely tied up with language; for explicit 
perception of the colour of an object - not of à coloured object - a | 
child has to be able to name colours. Red is the earliest to be named 
correotly, then blue follows, often as the opposite of red, e.g» non-red, 
Children are slow to abstract colour as an independent and variable 
quality (Vernon 1962, Ch. 6). 
Colour is mostly associated and remembered in connection 
with & particular object. Red, for instance, as the sun 
at sunset, as a rose, blood, traffic light; although 
these reds differ considerably, we generally have only 
one word for ite. 
Experiments indicate, that the exact hues of colour 
which possess simple and commonly accepted names are