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visually/ 93 and so, according to the way in which the particular eye is constructed, 
the effective centre of the lens may not coincide with its optical centre. Fortunately 
for photogrammetry, this discrepancy often tends to compensate for the coloured 
fringes due to the chromatism and tilt of the lens. 38,103 The sign and magnitude 
of any residual effect depend on the observer, but any remaining error could 
probably be reduced to insignificance in a particular case by the use of special 
non-deviating prisms of suitable chromatic dispersion. 
Every photogrammetrist knows the sensation of seeing in one comprehensive 
vision the space from the topmost mountain in his model right down to the valley 
floor. What is the physiological mechanism behind this ability? One possibility 
is that the eyes make rapid scanning movements in convergence in order to cover 
great ranges of depth. [4] This certainly occurs when the depth differences are very 
great indeed, but the rate of alternation is not then very rapid, and the observer is 
usually aware of concomitant sensations of refocusing and reconverging. [4,11,12] Very 
often, however, most of the objects of interest in the central field may fall within a 
parallax range covering less than, say, 20 minutes, and then the convergence angle 
may become remarkably steady, indeed fluctuating by only ±2 minutes, as though 
the observer were looking fixedly at some intermediate plane of depth.” 21 He can 
nevertheless see all the objects as single, provided that they do not fall outside his 
parallax range for single vision, which may extend in depth by more than 10 minutes 
in front of and behind the fixation point.” 31 (The range is greater for peripheral 
vision.) Such apparently small angles can represent surprisingly large distances in 
real space. For example, with steady convergence on a point 10 metres away, the 
observer could see all objects from about 7 metres to 20 metres in distance, without 
double vision and without having to alter his convergence. When converged at 
20 metres, a distance analogous to the hyperfocal distance of a camera lens, he 
could see objects as single all the way from 10 metres to infinity.” 21 
Now in order to explain this fusional process, we may suppose that for each 
point in one retina there is a corresponding point in the other, such that simultaneous 
stimulation of each pair of corresponding points gives rise to a single perceived 
image. 343 We then have to explain how the image on one corresponding point can 
be displaced on the retina by a distance representing more than 10 minutes of 
parallax (about 0-05 mm. or about 20 cone diameters), and yet still fuse with an 
image on the original corresponding point in the other eye, for this must occur 
whenever the observer fuses two objects widely separated in depth. The most 
convincing explanation is in terms of the manner whereby the neurons from corre 
sponding retinal elements meet in the visual cortex of the brain, with numerous 
cross-linkages, to form in effect a three-dimensional lattice. By these means, the 
retinal images, which are two-dimensional representations, of course, could be 
combined to give a cortical representation in three dimensions. In other words, 
a three-dimensional model of the outside world would be formed within the brain 
itself. 3143 The region of single vision would thus be limited by the thickness of the 
cortex, or by the extent to which the innervations could be diffused to neighbouring 
nerve networks. 
As another example of this fusional ability, a difference of up to 20 per cent 
in magnification between the retinal images can be tolerated with hardly any effect 
on the visual impression. 3153 This surprising facility probably arises from our 
having to deal with such situations in everyday life when looking at objects close to 
one side of the head. Obviously there must then be a difference between the sizes 
of the retinal images for which it would be very desirable to compensate. On the 
other hand, if the images are differently magnified in only one meridian, fusion can 
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