4 
in my view, changed the position somewhat* 
Hitherto the disadvantage of mono-measurement has been that it 
rests upon pre-marking of points on more than one picture. It is 
difficult enough to make a measurement to an accuracy of say +4p, which, 
by repetition, can be reduced possibly to + 2p., but to form a physical 
mark to this accuracy not only presupposes that a visual setting is 
made to this precision, but that the marking device does not add 
further errors. This is asking a great deal and one might even go so 
far as to say that measurement after physical marking, however good, 
is bound to be worse than direct measurement since physical marking 
must be preceded by visual setting. But the independent measurement 
of plates has two advantages, simplicity of organisation and simplicity 
of the measuring equipment; and any endeavour to overcome its basic 
defect should be taken seriously. 
Very briefly, J.A. Eden makes the point that the magnification 
under which stereoscopic measurements can be made is severely limited, 
perhaps to x 12 with good quality pictures. The reason he gives is 
that stereoscopic measurement depends upon the observation of all 
detail in the neighbourhood of the point of interest and that much 
of this detail has such low contrast that it disappears under higher 
magnifications or is, at any rate, not fusible with corresponding 
detail on the neighbouring picture. He noticed however that always 
(or nearly always) there appear to exist sufficient numbers of 
isolated high-contrast objects that can be identified with certainty 
on two or.more photographs and so small that they do not have to be 
marked, but simply identified in some unambiguous way under a magnifi 
cation of x 40. The subsequent measurement can be made in a mono 
comparator with great confidence under the same magnification of x 40« 
In his quoted paper Eden «seems more concerned with the problems 
of capital cost and organisation than with accuracy; and he makes the 
point that the prima facie practical advantages of mono-comparator 
measurement can almost certainly be realised without loss of accuracy 
if physical marking of the points is abandoned for his identification 
method. However I would not be surprised if further careful work 
were not to show that a significant improvement of accuracy was 
possible as well. At any rate this new suggestion ought to be 
pursued along with experiments into expensive and elaborate methods 
of physical transfer of points. 
The problem of photographic observation seems to have 
resolved itself into deciding between stereo- or mono-measurement * 
The latter has great attractions, but, at the moment, the advantage 
of accuracy seems to lie with the former, in spite of the basic 
instrumental advantage of the mono-comparator. I am strongly of 
the view that this problem is ripe for solution and should be 
actively pursued during the four years following the Lausanne 
.Congress. 
+ 
It is, of course, this that makes stereoscopic observation essential 
to plotting.