390 VISION THROUGH SMALL APERTURES IN WALLS. 
from P come to a focus, the observer will be able to see objects, as at 
O beyond the hole P. 
_ The image appears inverted, but by using two lenses, DZ and DL’ as 
indicated in figure 2, the image appears erect. 
Fie. 2. 
Sometimes assistance is derived from placing a second lens, shown 
in dotted lines in figure 1, close to the eye, and it is more convenient 
for adjusting the position of the eye to have a diaphragm at the eye end 
with a small hole near the focus J. : 
The eye-piece can be moved back and forth to focus the image of P 
on the point J for any position of Z. If Lis then nearer to L than P 
is, the object O will appear magnified. 
The observer has his eye im the image of the outer pinhole, and 
therefore looks through it freely. 
2nd. Reflecting Instrwment. 
Two mirrors M and M’, figure 3, are substituted for the one lens L 
Wits Sh 
of figure 1. If these mirrors are elliptical, and placed so that the point 
C (where rays cross) and the point P are foci of the ellipse correspond- 
ing to M', and if the foci of the other ellipse (either equal or unequal 
to the first) are at the points C and J, the image will doubtless be 
clearer than with spherical mirrors. Difficulties of construction have 
interfered with making the mirror instrument. 
A single piece of glass silvered at M and M' might be substituted for 
the two mirrors, as shown in figure 4. The refractions at the two 
plane surfaces would compensate each other. 
Such instruments give the means of looking through thick walls, as 
on the turret of an armoured ship or fort, and of obtaining a wide field 
