66 ROYAL SOCIETY OF CANADA 
frame B. Should the instrument be properly adjusted, the hole of 
the screen and the image of the hole of the eye piece seen by reflection 
in the mirror will coincide when looked at through the eye hole. 
The reflecting glass F gives a virtual image of the eye hole O in 
0,;,FO0,—FO. The mirror B gives another virtual image of O, in 
0,;,P 0, — PO, The reflecting glass F gives a third virtual image 
pr Oy m0 END 407 | 
O3 

ES 

Fig. 2. 
It is not necessary that the transparencies be vertical or parallel 
to the axis of the stereoscope or parallel to each other. The only 
condition to be fulfilled is that the visual ray parallel to the axis, 
that is to say the line passing through the principal point of the pers- 
pective, be reflected by the glass plate perpendicularly to the plane 
of the transparency. This line can be moved to any direction by 
means of the adjusting screws, therefore it can always be made per- 
pendicular to the transparency. A frame may be placed at B’, which 
is parallel neither to B nor to the axis. 
Should the two holes not coincide when seen through the eye 
hole, they are brought together by means of the adjusting screws of 
the glass plate F. The other glass plate is adjusted in the same man- 
ner. The instrument is now ready for plotting. 
The first thing to be done is to mark the extremities of the 
horizon and of the principal lines upon the transparencies which 
are then inserted in their frames. The eye holes plate is set at 
