394 VISION THROUGH SMALL APERTURES IN WALLS. 
By having the whole apparatus revolve around the line [ P while 
the mirrors also rotate around the axis C, a round hole might be 
substituted for the slit at Pand an “all-round” image be obtained, 
as with the telescope. 
The same arrangement of mirrors can be used for delicate measure- 
ment of angles, and by its use an angle can be trisected optically in 
the field. 
When the instrument is used for measuring angles, a large aperture 
with cross-hairs at the point P can be substituted for the slit. 
Ist. Use of Instrument for delicate measurement of angles. 
If the line J P (in figure 5) is set in any direction, the mirrors can 
Fia. 5. 
be turned to view an object in some other direction, and the angle 
between the two lines of direction will be less than the angular 
displacement of the mirrors from positions perpendicular to the line 
I P, where the image coincides in direction with the line of direct 
vision through the slits. 
Let R represent C [=C P, and r represent C d=C d’. 
Let a represent the angular displacement P C d of the mirrors from 
positions perpendicular to P J, and 2 the angular deviation P I m, 
from the direction I P, of the image seen from J. 
The angle d C m=t I m=a—x, 
