VISION THROUGH SMALL APERTURES IN WALLS. 893 
Part of the side of the box shown in this figure is cut away to show 
the shaft on which the plate revolves. It can project below the box, to 
be turned by hand, or it can be revolved by a small electric engine or 
other mechanical contrivance. ‘The space over the centre of the plate 
should be unobstructed. 
If a diaphragm perpendicular to the two mirrors is set up between 
them, having a slit over the axis, the walls for apertures P and J may 
be dispensed with. ‘The eye may then be placed at any distance from 
the axis for observation. The position of the diaphragm is indicated 
in dotted lines in the perspective view, figure I. 
Figure 2 represents the device in plan. ‘The course of the rays is 
shown in broken lines. A. second position of the mirrors is shown in 
outline. ‘The broken and dotted line shows the course of direct vision 
through the apertures. 
With this arrangement of mirrors the apparent horizontal size of 
the object seen from J is the same as if the eye were at P. 
A revolving prism, silvered at MZ and M’ (figure 3), might be substi- 
Fie. 3. 
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tuted for the mirrors. The cross-section of the prism would be a 
parallelogram, and the refractions at the two unsilvered surfaces would 
compensate each other. 
This contrivance also allows observation through thick walls, with 
but a small aperture in the front face, but the wall will have to be 
hollowed out more than for the instrumeut with fixed mirrors or 
lenses. 
Higure 4, representing a section of the wall, shows that the eye can 
Fig. 4. 
be readily protected from shots through the outer opening, Compare 
figures 5 and 6 of Part I. 
