92 SECTIONAL ADDRESSES. 
photographie vision a new sense. For I do not exaggerate when I use those 
words—a new sense—to describe the power which the stereoscopic measure- 
ment of pairs of photographs has given the surveyor of topographical 
detail. Colonel Laussedat in France, Mr. Deville in Canada, were pioneers 
in the simple measurement of photographs, searching for pairs of recognis- 
able points, and deriving their distances and heights by a tedious compu- 
tation or construction. The brilliant idea that at least part of this could be 
done automatically was due to Captain Vivian Thompson, an assistant 
instructor in the Survey School at Chatham ; and I well remember seeing 
his process in embryo when I was first introduced to the pleasant delights 
of survey by my god-father in that art, Sir Charles Close. But Thompson’s 
machine was lamentably poor in construction; and those who used it 
seem to have loved it little. 
The credit of extending and perfecting the beautiful but marvellously 
simple geometry belongs to an Austrian, Lieut. von Orel; and of translating 
the geometry into sweetly-working mechanism to the German firm of 
Zeiss. No more beautiful piece of optical machinery has ever been made, I 
willingly believe, thanthe Stereoautograph of von Orel and Zeiss. But they 
made a sad mistake in marketing it, granting exclusive rights over a terri- 
tory to an individual, and demanding from him not only ‘a heavy price 
for the outfit, but a large and perpetual royalty on his gross receipts as a 
stereoautographer. So strange a method of selling a scientific instrument 
was never known before. Imagine the plight of the stereoscopic surveyor 
in Tibet confronted with an injunction obtained by the Concessionnaire 
for Nepal if he dared photograph the South Peak of Mount Everest. 
Happily there is more than one mechanical and optical solution of the 
problem, and at least four different machines are now in the field abroad, 
while a fifth is under construction in this country to the order of the Air 
Survey Committee. We are thus fortunately saved from the reproach that 
nothing has been done in this our country to develop a method first 
devised by an Englishman. But we may feel that our instrument-makers 
and our surveyors have been a little unenterprising ; and there is nothing 
I would like to see more than a real effort, with adequate means, to try out 
stereographic surveying on geographical scales. 
We know well enough that the stereoautograph can deal marvellously 
with a small piece of country on a large scale; but what has never yet 
been shown is that it can deal with a large piece of country on a small 
scale. It will contour for you an inaccessible cliff at 1-metre intervals on 
the scale 1/5000 ; but can it or a rival be made to tackle 100-metre intervals 
on the scale 1/250,000? That is a question which has never yet been 
answered, and I believe that it is our duty to answer it. Along the northern 
frontiers of India in the ranges of the Himalaya are at least 10,000 perma- 
nently snow-clad peaks. I have heard from the lips of an Indian Survey 
officer the deplorable remark that a certain rather rough method was 
“ good enough for the mountains.’ No true geographer would admit that 
anything short of the very best is good enough for the grandest mountain 
region in the whole world ; yet it is easy enough to see that the surveyor 
had a certain justification. So long as the inch-to-the-mile map is in- 
complete in the plains, we can hardly expect it in the mountains. Yet a 
really accurate map on the scale, say,1/250,000, cannot be deemed superfluous 
for defence ; and even the poor geographer or traveller is entitled to ask 
