MEASUREMENT OF OBJECTS. 
thickness of the glass which covers the scale; and however thin 
this glass may be, where high powers are used, it will render the 
difference of the foci of the scale and the object so sensible, that 
they can never be both seen with sufficient distinctness at the 
same time. 
57. We know no greater example of the inexhaustible resources 
of art, and the untiring zeal with which its cultivators minister to 
the wants of science, than the wonderful perfection to which the 
mechanical division of a material so fragile as glass has been 
carried. For the reasons we have here stated, as well as because 
in the application of the highest magnifying powers the object- 
glass of a microscope requires to be almost in contact with the 
object, without actually touching it, microscopists required 
extremely thin plates of glass to cover delicate objects mounted 
on their slides. Messrs. Chance of Birmingham responded to this 
demand by the production of plates of glass so thin, that three 
hundred of them piled one upon the other are no higher than 
an inch. 
For examples still more striking of the minuteness with which 
lines may be traced upon glass by mere mechanical processes, we 
may refer the reader to that part of our Tract upon Microscopic 
Drawing and Engraving, in which the test plates of Mr. Nobert 
are described. 
58. One of the most evident expedients for the measurement of 
microscopic objects would seem to be the micrometer screw, which 
is applied with so much success, and with results of such extreme 
precision, in astronomical instruments. Yarious methods of apply¬ 
ing it to the microscope will suggest themselves to every one who is 
familiar with its uses in the observatory. Let two filaments of 
spider’s web, or micrometric wire, be extended at right angles 
across the field in the focus of the Fig. 25. 
eye-piece. These will divide the field 
horizontally and vertically at right 
angles, intersecting at its centre, as 
shown in fig. 25. Now suppose 
the stage supporting the object is 
capable of being moved by a micro¬ 
meter screw, having for example 
one hundred threads to the inch. 
Let the object be placed first so that 
its length shall be horizontal, and 
let the slip be adjusted so that 
the vertical micrometric wire b V 
shall coincide with one of its extremities. Let the micrometer 
screw be now turned so that the object shall move horizon- 
53 
