VISION THROUGH SMALL APERTURES IN WALLS. 897 
Tuirp Form. 
In this, one mirror M’ is fixed, and the second one is movable, the 
latter remaining, as before, always parallel to the former. See figures 
8 and 9. 
Fie. 8. 
In figure 8 the motion of the mirror M is perpendicular to itself, and 
in figure 9 it is on a line inclined to its face, the inclination, however 
not exceeding that of the line JP to the mirrors. A second position 
of the movable mirror is shown in outline. The mirror oscillates back 
and forth from a position continuous with the fixed mirror. 
By having a succession of movable parallel mirrors their motion can 
be made continuous instead of oscillating, giving greater uniformity of 
illumination to the image. 
When the fixed mirror is midway between the apertures, the point 
of reflection on the movable mirror will be constant, and its position 
will be in a line passing through the middle of the line P J, and 
perpendicular to the mirrors. 
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