102 Tue Mrcroscopr. 
1. Simplicity, as compared with the screw fine adjust- 
ment. <A screw adjustment is a double faced projecting spiral 
inclined plane wound on a shaft. This plane runs on another 
re-entrant double spiral inclined groove winding around the in- 
side periphery of a hole or shaft usually fixed. For use on the 
microscope stage the screw adjustment must not wabble, yet it 
must move readily and have no loss of motion upwards or down- 
wards, inwards or outwards. Experience has taught me that it 
takes a skilled mechanic to make a good fine screw adjustment. 
For ordinary screw threads the requirement is to bind in one 
direction but notin the other direction. Such screw threads illy 
answer for moving backwards and forwards with the accurate 
delicacy of such an instrument of precision as the compound 
microscope. It is then expensive to make a fine screw adjust- 
ment and there are few workman that can make them. 
On the other hand the cam adjustment is easily made by 
centering a metallic disc, outside of the true center, on an 
axis of steel wire. It is simple to mount. Even an unskilled 
artisan can make and mount it. 
2. Cheapness.—Nickel five cent pieces may answer for 
cams. These have a smooth rim and a good place to bore a new 
center in the upper part of the shield on the “In God we 
trust ” side of the coin. 
Besides it is inexpensive to turn out a like disc on a lathe. 
3. Effectiveness—No mechanical motion is so sure and ef- 
fective as that of acam. A short lever attached to the axis of 
the cam gives the means of applying required motions with ease 
and certainty. The amount of motion can be regulated ex- 
actly. It is rapid and sensitive. What more could be asked 
for a fine adjustment ? 
4. Not liable to get out of order as the spring or springs 
holding the stage to the stage plate keep the parts in contact 
together, and compensate for the loss by wear which is on one 
surface in one direction, to-wit, downwards; while the wear of 
the screw is on two surfaces in two directions, to-wit, upwards 
and downwards. In the hands of an active worker this wear of 
a screw makes an unpleasant loss of motion in two directions, 
and which it is not easy to remove in the case of a screw. I 
‘ia 3 
