[The eye possesses a Contrast Sensitivity, the emulsion cannot 
distinguish between two levels of grey, it can only record 
passively intensities over a relative large range (1:100). 
A man can decide about equality of two intensities, and note very 
small differences, of the order of 1% or even somewhat smaller. 
These primary activities are executed on the way from retina to 
central brain, the retina is adapted to this type of judgement, 
and not to absolute estimations of intensities. 
the emulsion has a certain granularity; the eye's granular 
retinal structure is largely compensated for by involuntary 
vibrations and scanning movements. 
Only in spectral sensitivity a photographic emulsion can compete 
e E with the eye: all emulsions are sensitive to ultra violet, some 
even to infrared. Still the eye has a normal outfit three 
different types of rods for the three wavelength ranges, whereas 
the standard emulsion is a panchromatic film, and not the 
expensive colour film. | 
This list shows the interesting properties of the "camera' section 
of the diagram. Here too man has since a century tried to replace 
the eye with a technical piece of equipment, but this has resulted 
in a wider use of the eye, because thanks to the photographic 
technique, we can see in the past and on places where the eye never 
could function. Considering the list once more, the author is aware 
that also he discusses the eye in comparison to the camera - it is 
true that we know a camera better. 
  
   
  
  
  
  
  
  
   
  
  
  
  
   
9 The Biological Cornerstone of Photo-interpretation (Fig. 8, top) 
  
To be consequent with the line followed so far, we now should pay 
attention to the upper and lower corners of the matrix; the places 
where the physiological and psychologica properties merge. 
These corners do not have technical equivalents, ihey belong 
primarily to the typical biological and human domain. In short, they 
represent the machinery of nature, using Bronowski 's terminology 
(1965). 
The Biological Aspects of this machinery are easily demonstrated 
by the property "Visual Fatigue" — the most important human remote 
sensing equipment will get tired after a couple of hours, even if it 
is not overloaded, whereas & Nimbus satellite camera works continuously 
for years. 
PhotoInterpretetien