MEASUREMENT OF OBJECTS. 
Since they are equally magnified, their real will he in the propor¬ 
tion of their apparent magnitudes. If, therefore, they appear 
-equal, they will be equal, and if that which we desire to measure 
appear to be twice or half the size of that whose magnitude we 
know, its real magnitude will be twice or half that of the latter. 
Such was the micrometric method used by the earlier observers. 
Thus Lewenhoeck procured a number of minute grains of sand, 
•sensibly equal in magnitude, and placing as many of them in a 
line, and in contact, as extended over the length of an inch, he 
ascertained the fraction of an inch, which expressed the diameter 
of each. "When he desired to ascertain the actual magnitude of an 
object seen with his microscope, he placed one of these grains 
beside it, and estimated by comparison the magnitude of the 
former. 
Various natural objects, whose magnitudes are known, and 
which are subject to no perceptible variations, such as the sporules 
•of Lycoperdon bovista or puff-ball, whose diameter is the 8500th 
of an inch, those of the lycopodium, which measures the 940th 
of an inch, and others such as hair, the filaments of silk, fiax, 
and cotton, and the globules of blood, have been suggested as 
standard measures to be similarly used. 
More modern observers, adhering to the same method, have 
substituted artificial for natural standards. Thus extremely fine 
wire, called micrometric wire, has been used. This wire can be 
drawn with an astonishing degree of fineness. Dr. Wollaston 
invented a process by which platinum wire was produced, whose 
thickness was only the 30000th part of an inch. * 
56. Such measurements are now more generally made by means 
•of a minute scale engraved on glass, with a diamond point. Let 
us suppose, for example, a line, the 20th of an inch in length, 
traced across the centre of a glass disc, set in a thin brass plate 
of the size and form of the sliders on which objects are mounted. 
Let this line be divided into 100 equal parts, every fifth division 
being distinguished by a longer line, and every tenth by a still 
longer one. Each of these divisions will be the 2000th part, 
the intervals between the fifth divisions will be 400th, and that 
between the tenth divisions the 200th part of an inch. This 
microscopic scale will be seen magnified with the microscope, and 
any microscopic object laid upon it will be seen equally magnified, 
so that its dimensions can be ascertained by merely counting the 
divisions of the scale included between those which mark its 
limits when placed in different positions on the scale. 
It may perhaps be thought impracticable to make divisions so 
* Handbook of Natural Philosophy, 2d edition, Mechanics, 38. 
e 2 51 
