MICROMETERS. 
zero of the scale by turning the head l so as to cause the screw to 
retire from the piece e e , the spring f then pressing this piece to¬ 
wards the screw. The object to be measured is then inserted 
between the end of the rod projecting from e and the screw, and 
consequently the piece e e and the index line engraved upon it 
will be pushed from left to right through a space equal to the 
thickness of the object. This thickness may then be ascertained 
by observing with the microscope the division of the scale to 
which the indicating line has been advanced. 
60. A micrometer, having some resemblance to this, but made 
more applicable to the general purposes of microscopic measure¬ 
ment, has lately been contrived by Mr. Jackson, a description of 
which is published in the “Transactions of the Microscopical 
Society.” 
A disc of glass, upon which a micrometric scale is engraved, is 
set in a thin plate of brass, which moves with a sliding motion on 
another plate, in which a corresponding hole is made. The former 
is like that of M. Le Baillif, urged by a fine screw in one direc - 
tion, and driven back by a spring in the other, as shown in fig. 24. 
This micrometer slide is inserted in the tube of the eye-piece by 
openings in the sides of the tube, as shown at m in fig. 27, which 
openings can be closed when the micrometer is not used by sliding 
covers, as shown at a, fig. 28. 
It is easy to see how this contrivance is applied. The scales 
magnified by the eye-glass are projected upon the optical image of 
the object produced by the object-glass, and this image may be 
made to move so as to bring its extremity to coincide with the first 
division of the scales. The scale will then show not only the dimen¬ 
sions of the entire object, but those of its parts. The object may 
be turned in any direction relatively to the scale that may be 
desired, by means either of the hand or the stage adjustments. 
55 
