VISION THROUGH SMALL APERTURES IN WALLS. 3891 
of vision without having a large opening in the front face exposed to 
fire. 
Figure 5 shows at A the horizontal section of such a wall. At B 
Fie. 5. 
and C are shown sections of usual construction, with the peep-hole 
_ flaring towards the enemy, or the reverse, the latter construction 
requiring the observer to keep moving his eye from point to point 
for observation. 
With the instrument described the same effect is obtained without 
moving the eye. 
Figure 6 shows that with a double reflection of the rays by two 
Fria. 6. 
additional, stationary, plane mirrors, N and N’, the eye can be protected 
from shots through the outer opening. 
The same arrangement would allow of sending a divergent beam of 
light through thick walls from within, without a wide opening in the 
walls. 
Such an arrangement could be used also for making enlarged images 
in the pinhole camera-obscura without great increase of depth. By 
placing a “dark box” inside the wall of a turret, the image of a sight 
outside could be made to coincide with the image of the target, seen 
erect as at fin figure 1. 
Use might be made of the telescope for inspecting the bore of 
cannon, and also whenever the observer needs to remain concealed 
from view. 
Fig. 7. 
