MICROMETRIC SCALES. 
Fig. 
V 
even and uniform that no irregularity in their edges is discover - 
able by any microscopic power to which they are submitted. In 
the process of tracing the divisions, the point is moved over the 
glass, the latter being fixed, or the 
glass moved under the point by means 
of a very fine screw, called a micrometer 
screw, the magnitude of the thread of 
which is exactly known. The head 
of this screw is a metallic disc, fig. 3 ; 
the circumference of which is divided 
into from 200 to 400 equal parts, or 
even into a still greater number. 
Let us suppose, then, the screw to be 
so fine that there are 50 threads to an 
inch, and the circumference of its head 
is divided into 100 parts; one revolu¬ 
tion of the head will therefore move the 
screw and the diamond point upon which it acts through the one- 
fiftieth part of an inch. But if a fixed index he directed to the 
circumference of the head, so that the motion of the head through 
one division can he observed, such motion will move the diamond 
point through the 1-5000th of an inch. 
The cutter, after tracing each division, is raised from the glass, 
while the diamond point is pushed forward by the screw to the posi¬ 
tion necessary to engrave the next division of the scale, and proper 
mechanism limits the motion of the screw, so as to regulate the 
relative lengths of the divisions of the scale in the manner already 
explained. 
9. A scale thus engraved, being viewed with a microscope whose 
magnifying power is proportionate to its minuteness, the divisions 
are rendered as distinctly visible as those of an ordinary rule are 
to the naked eye, and if the object to he measured be laid upon the 
glass its dimensions may be ascertained, as those of an object of 
ordinary size would be by a common rule. 
10. These scales vary in the magnitude of their divisions, ac¬ 
cording to the magnitude of the objects which they are intended 
to measure. On those which have the largest divisions, an inch 
is divided into 500 parts; scales, however, are furnished by the 
opticians for microscopes in which an inch is divided into 2500 parts. 
11. However minute such scales may seem, they are by no means 
the most minute that have been executed. Mr. Froment, whose 
apparatus for the division of astronomical instruments is well 
known, has supplied me with a scale in which a millimetre is di¬ 
vided into 1000 equal parts. Each division of this scale is, 
therefore, onl^he 1-25000th part of an inch. 
53 
