THE MICEOSCOPE. 
tally. It will appear to pass gradually under the vertical wire 
until its other extremity shall coincide with that wire. If then 
the number of complete turns, and parts of a turn of the screw 
be counted, the length of the object then will be known. Thus, 
if at the end of every complete turn, the screw produce an 
audible sound like the tick of a clock, the observer can count 
the complete turns, and if the circumference of the head bo 
divided into 100 parts, and that an index be fixed upon the 
stage to indicate the position of the head at the commence¬ 
ment, the decimal parts of a turn can be ascertained, each division 
of the head corresponding to the 100th part of a complete turn, 
and therefore to the 10000th of an inch. 
By turning the stage so that the screw will cause the object 
to move across the field in the direction of the vertical wire, its 
dimensions in the other direction can be ascertained. 
59. A simple and ingenious micrometer for ascertaining the 
dimensions of such objects as would bear a slight pressure without 
change of form, was invented by M. Le Baillif. A plan and 
vertical section or side view of this are shown in fig. 26. 
Two upright pieces, c c, are fixed in a slip of copper, formed 
like one of the slides, having a circular hole in its centre, in 
Fig. 26. 
which is set a plate of glass, on which a scale o is engraved. 
Upon this is placed a moveable piece, e e , having a similar hole 
and plate of glass, with a fine line engraved upon it at right angles 
to the scale, so that when it is moved from left to right this fine 
line will coincide necessarily with all the divisions of the scale. 
Prom this piece, two rods proceed, which pass through holes in the 
upright pieces c c , and one of them is reacted upon by a piece of 
watch-spring, while the other abuts against the end of a fine 
screw, /, which moves in a nut, d. 
When an object is to be measured, the index line upon tho 
upper glass disc is brought to coincide with the first division or 
54 
