68 ROYAL SOCIETY OF CANADA 
For the sake of simplicity, it has been assumed that the two views 
of a stereoscopic pair were taken in a vertical plane parallel to the 
base line. This condition can be dispensed with and use made of 
views taken from the extremities of the base line 8 WS (fig. 3) in 
the directions S X and S Y forming with the base angles a 
and #. The adjustments of the stereoscope are made as usual, but 
with this difference; instead of placing the tracer upon a parallel to 
the axis of the stereoscope passing through the eye hole, it is placed 
in t (fig. 4) upon a line Ot, forming with the axis an angle equal to 




X Y ert 
/ | 
| 
| LA 
B | B 
| OL A / , ’ 
S| Pal ae (ite dal Wipe ANS i P 
x | 
Pete en oO (oy 
Fig. 3. Fra. 4. 
90°— a. | With such views, the reflecting glass plates have to be 
adjusted for each pair of views; it would, therefore, be proper to 
substitute ordinary tangent screws for the adjusting screws. The 
adjustment of the reflecting glass plates is a quick, precise, and simple 
operation; its repetition for each pair of views is a small matter and 
cannot cause any inconvenience. The vertical axis around which each 
reflecting glass plate revolves must be so placed that the focal distance 
of the transparency does not change when the glass is revolved by 
the adjusting screw. 
This property of the stereoscope permits the employment of views 
taken with cameras, such as the one used on Canadian surveys, which 
are not provided with telescopes for directing them accurately. There 
is little doubt that the difficulty of the stereoscopic method will be to 
obtain the pairs of views on the ground. The methods hitherto pro- 
posed require views taken perpendicularly to the base line; that seems 
simple enough, but when tried in the ordinary course of surveying 
operations, it may not be found so easy as it appears. It is an impor- 
tant advantage of the stereoscope that this condition has not to be 
fulfilled. ; 
