FHYSICS: A. G. WEBSTER 
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the small concave mirror 5, the back of which constitutes its own short lever, 
being directly pushed by a point 16 carried by the spider 13, which carries 
the disk 4. Instead of being pivoted in jewels, the mirror is carried by a 
torsion strip 9, cut from thin sheet steel with a square side projection upon 
which the mirror 5 is cemented. The strip is held by end clamps, and its 
tension may be adjusted by a screw 11, bearing on the spring 10, (inside). 
Two micrometric adjustments are secured by the strip being carried on a 
rocker 3, pivoted on two screws 6, allowing of sidewise displacement in order 
FIG. 2a 
to change the lever arm of mirror 5, and thus the magnification of the motion. 
The motion of the image sidewise in the field of the ocular is obtained by a 
slow motion of the rocker, controlled by a screw at its lower end, accessible 
from outside. All other adjustments of the image are made at the ocular end. 
With the usual leverage of one-fourth to one-third of a millimeter between 
the point and the axis of the mirror, and a distance of forty centimeters to the 
reticule in the ocular, a magnification of about 2400 is obtained, and as one 
can read to one-tenth of a milHmeter on the reticule, we may detect a dis- 
