3892 VISION. THROUGH SMALL. APERTURES IN WALLS, 
PARI ILil 
Mechanical devices for obtaining an extended view through a small hole 
or slit by means of two parallel plane mirrors. 
First Form. 
Two plane mirrors, Mand M’' (figures 1 and 2) are placed on a 
186, Al, 
Fie. 2. 
revolving plate S and perpendicular to it. 
They are at equal distances from the centre C, and placed sym- 
metrically with reference to it at opposite ends and sides of the 
diameter perpendicular to them. 
As the mirrors revolve, the rays from the object at O cross at the 
aperture slit at P, and are successively reflected to the eye at J for 
different positions of the mirrors, P and J being in line with the point 
C and at equal distances from it. A round hole will answer at J. 
All the rays which reach [ must evidently pass over the point C. 
Revolving the plate rapidly gives a continuous image within the 
range of the instrument, 
