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REPORT OF COMMISSION VII 73 
resulting in increased shadow from the grass, etc. Accordingly the barbed wire 
belts will show up as lines of shadow or contour-less narrow bands of a darker 
tone than the ground on either side (see Figure 14). 
As mentioned earlier, entrenchments, etc. are usually plainly visible in verti- 
cals, thus often making possible taking very accurate measurements of the 
dimensions and contours of gun emplacements. Through previous knowledge 
of the enemy's standard emplacement models, there is no difficulty in establish- 
ing the type of weapons for which the different earthworks are intended. 
Furthermore, grouping gives a good clue to the identification of smaller arms— 
such as mortars—and also to the position in the trenches of heavy infantry weap- 
ons. 
Aerial photographs therefore constitute the best possible foundation for 
the meticulous study of the enemy lines, and even of whatever changes that 
may take place in them; this study is always imperative, both for the attacking 
and the defending forces. 
EQUIPMENT 
INSTRUMENTS AND MATERIAL FOR PHOTO-INTERPRETATION 
The following instruments and material are used for photo-interpretation: 
stereoscopes, stereomicrometers, "loupes" and magnifying glasses of various 
kinds and power, precision measuring scales, pantographs, sketch masters and 
other drawing apparatus of different kinds, projection equipment and equip- 
ment for examining transparencies, etc. and effective lighting sources for the 
interpreters. 
The stereoscopes are of two kinds: a smaller type, preferably with a movable 
eye base, and a larger type, a so-called mirror-stereoscope, for studying larger 
photos. The distance between the center point of the lenses in the mirror- 
stereoscope is lengthened through prisms and mirrors to 30-40 cm. If one wishes 
to examine details with the help of greater power, monocular prism-glasses 
are placed over the stereoscope ocular, generally magnifying 3-4 times. To 
be able to move the mirror-stereoscope easily across larger photo surfaces with- 
out touching them, it is best to provide them with a special arrangement for 
parallel moving. Improved stereoscope constructions, permitting more conven- 
jent studying of individual parts of the photo and also giving varying degrees 
of power, are desirable. 
Improvement is also desired in the present pocket type stereoscope so that 
different powers may be used with minimum effort. 
With the help of stereomicrometers and the drawing apparatus which be- 
longs to them, the necessary simple stereophotogrammetric measurements 
are carried out, such as establishing relative height and dimension, drawing of 
single elucidating level curves, etc. 
When the stereomicrometer is built together with the mirror-stereoscope, 
an instrument of greater precision is obtained, as an example, stereo-compara- 
tors arranged for field use. 
For especially accurate measurements, high quality automatic stereo 
instruments are used. 
Binocular loupes, worn on the head are very suitable for interpreting single 
photos. They protect the eyes from the dazzle of direct lighting, and permit 
work with a certain amount of power without eyestrain. Loupes of 5 dioptrics 
are most suitable. With similar help it is possible to interpret photos over a 
long spell of time and to retain accuracy. 
'The magnifying glasses should be aplanatic, and the loupes of good quality. 
Aplanatic loupes which magnify about four times, have a diameter of 4-5 cm.