[DEVILLE] WHEATSTONE STEREOSCOPE IN SURVEYING 65 
interval of the eyes. This interval being variable with individuals, 
means must be provided to adjust the distance of the holes when the 
instrument is to be used by more than one person. The adjustment 
can be made by a simple arrangement, not shown on the figure. 
Behind the eye holes are two vertical glass plates with parallel faces, 
set at an angle of 45° with the axis. The front face is slightly 
silvered, but not enough to prevent seeing a bright point through the 
plates. Each one is provided with two adjusting screws like the 
small mirror of a sextant. One of the screws, G, revolves the glass 
around a vertical axis; the other screw, not visible on the figure, 
moves it around a horizontal axis parallel to the plate. 
The transparency frames are upright and in the direction of the 
axis of the apparatus. They can be moved up and down by the screws 
HHH, or right and left by the screws JJ. They are maintained 
in their planes by pins sliding in the slots K. For transparencies 
printed by contact, the distance from a frame to the image of the 
corresponding eye hole seen in the glass plate must be equal to the 
focal length of the objective used for taking the photographs. A 
better arrangement is to place the frames at the most convenient dis- 
tance and to produce the transparencies by enlargement from the 
negatives in suitable proportion. 
The base of the tracer C consists of three arms supported by 
foot screws O, the head of the screws being of spherical shape. The 
central arm carries a pencil N and two upright posts upon which 
slides a rectangular plate or screen L painted dead black. A small 
hole is pierced in the centre. The point of the pencil is in the 
plane of the face of the screen. The height of the screen is indi- 
cated by a scale MM upon one of the posts; when at zero of the 
scale, the hole of the screen is at the same height as the eye piece 
axis. The graduation extends on both sides of zero. 
Before using the instrument for plotting, it is necessary to 
adjust the tracer and the reflecting glass plates. 
For adjusting the tracer, move the screen to the top of the posts 
and mark the middle of its upper edge. The mark must not move when 
the tracer is revolved around the axis of the pencil, held stationary. 
If found to move and describe a circle, it is adjusted by means of 
the foot screws OO. 
To adjust the glass plates of the viewing stand, set the line of 
the eye holes horizontal by placing the vernier at zero of the graduated 
arc, fix the tracer screen at zero of the altitude scale, draw on the table 
the trace Ot, fig. 2, of the vertical plane parallel to the axis and passing 
through one of the eye holes, the left one, for instance, place the 
point t of the pencil upon this trace and insert a plane mirror in the 
