[DEVILLE] WHEATSTONE STEREOSCOPE IN SURVEYING 69 
The scale of the plan is the proportion between the interval of 
the eye holes and the distance (not reduced to the horizon) between 
the two stations. Should, for instance, the holes be 65 millimetres 
apart, and the distance of the stations be 65 metres, the scale of the 
plan will be 1/,560- 
The direction of the meridian is deduced from the azimuth of 
the axis of the camera, represented upon the plan by a parallel to 
the axis of the stereoscope. 
The plan is afterwards reduced or enlarged by photography to 
the scale of the general map. It will be observed that after the 
views are taken, the scale of the plan drawn by means of the stereo- 
scope is invariable, depending only upon the distance of the stations; it 
is, therefore, essential to select this distance so as to obtain a suit- 
able scale. The plan should not be so small that details would 
become indistinct, neither should it be so large as to extend beyond 
the range where the tracer can be handled conveniently. This 
limitation in the selection of stations is a disadvantage and the 
instrument is inferior in that respect to the stereo-comparator, with 
which the distance of the stations may vary to a considerable extent. 
It is true that other organs might be introduced in the stereoscope 
for changing the scale of the plan, but the instrument would become 
more complicated. 
It must also be noted that the scale is the proportion between 
two distances, one of them being the interval of the eye holes which 
is liable to variations according as the eyes are opposite one part or 
another of the holes. Moreover, no magnifying power is available 
and there are other causes of error. So far as accuracy is concerned, 
the stereoscope is, therefore, far inferior to the stereo-comparator. 
On the other hand, the plotting is much less laborious and the instru- 
ment would seem to be particularly suitable for detailed reconnais- 
sance surveys. It may be that unforeseen difficulties will be met with 
in practice; until the instrument has received the test of practical 
experience, it would be idle to speculate upon its value. 
