52 PHOTO-MICROGRAPHS. 
ratus. Here, within easy reach of the operator’s 
fingers, it is fastened to a spool, around which it 
passes several times. This spool is made to re- 
volve with sufficient friction to resist the action 
of the weight, and by means of it the operator is 
able to move the adjustment screw with great 
facility and precision. 
The weight should be just sufficient to move 
the adjustment screw, and it will be found that a 
small bag containing shot (the proper quantity to 
be ascertained by experiment) will answer the 
purpose very well. Attach a metal hook to the 
bag of shot, and a ring to the end of the cord, 
so that the weight can be conveniently detached 
when the apparatus is not in use. 
The diameter of the spool by which the move- 
ment for focusing is effected should be considera- 
bly less than that of its elevated margin, so that 
the operator may have the advantage of leverage 
in raising the weight. One inch for the axis, and 
two and a half to three inches for the margin, will 
be a proper size. The spool is to be turned out 
of wood or metal, and is fastened to the support 
by means of a single screw, around which it re- 
volves. The necessary resistance to its revolution 
from the pull of the weight is effected by simple 
friction; or, better still, by the introduction of a 
coiled spring between the upper extremity and 
the board to which it is attached. 
The fine adjustment is to be brought within 
reach in the same way; but this is more easily 
moved, and a simple groove around the milled 
