SECTION III., 1902 [63] Trans. R. 8. C. 
VI. On the use of Wheatstone Stereoscope in Photographing Surveying. 
By E. DEVILLE. 
(Read May 27, 1902.) 
Dr. C. Pulfrich, of the Carl Zeiss Optical Works, in Jena, has 
devised an instrument, called stereo-comparator, for the purpose of 
making accurate measurements on stereoscopic views. The instru- 
ment, which is described in “ Zeitschrift fur Instrumentenkunde,” 
Nos. 3, 5, 6, and 8, of 1902, is constructed by the firm of Carl Zeiss; 
it is perfect in every detail and its results are remarkably accurate. 
Among other uses, Dr. Pulfrich describes its employment for photo- 
graphic surveys. Taking two views of the ground from properly 
selected stations, the stereo-comparator gives, by means of a micro- 
meter screw, the distance of any point of the landscape. The 
direction of the point is afterwards taken from the views by any 
of the well known methods and transferred to the plan upon which 
the point is then located by its distance. The altitude is deduced 
from the distance in the usual way. ‘The instrument appears to be 
thoroughly practical and may prove of great value in surveying. 
A little difficulty may be experienced at first in securing proper 
stereoscopic views, but Dr. Pulfrich has already indicated the prin- 
ciple of a camera for that purpose and it may be hoped that the 
Carl Zeiss Works will soon place an efficient instrument at the dis- 
posal of topographers. 
Another solution of the problem of stereoscopic surveying 
occurred to the writer, and experiments were commenced in 1896; 
owing to pressure of other duties, they had to be abandoned. In 
view of the attention now given to the subject, a description of the 
instrument devised at the time may offer some interest. 
The apparatus is a Wheatstone or reflecting stereoscope provided 
with such adjustments as are necessary for plotting topographical 
pians. Let it be assumed that the two photographs of a pair are taken 
in the same vertical plane, that is to say, that the plates are vertical 
and the optical axis of the objective perpendicular to the line joining 
the two stations from which the exposures «are made. The photo- 
graphs may be taken with one camera carried from one station to 
the other, or, preferably, with two identical cameras operating simul- 
taneously. 
The instrument, fig. 1, consists of an eye piece or viewing stand, A, 
with two eye holes, DD, of two frames, BB, for the transparencies, 
