MICROMETRY AND THE CAMERA LUCIDA. 53 
difficult to measure small objects, since, as mounted on 
the slide, they tend to lie obliquely across the scale, and 
their edges would only rarely coincide with its lines. 
As a rule, therefore, some indirect method of measure- 
ment is adopted. 
2. The Ocular Micrometer.—The most satisfactory 
method of measurement with the microscope involves 
the use of another scale, called the ocular micrometer. 
This is a circular piece of glass cut to fit in the eyepiece 
of the microscope and bearing a scale made up of lines 
at an equal, but not necessarily a known, distance apart. 
The micrometer is to be placed inside the eyepiece, 
resting on the sliding diaphragm inside it, which should 
be so adjusted as to lie in the plane of the real image 
formed by the objective. Thus ‘the image will lie on 
the scale, and may be measured by it. The value ob- 
served is therefore the size of the virtual image of an 
object formed by the objective in terms of an arbitrary 
scale. In order to obtain actual dimensions the ocular 
micrometer must be standardized in order to determine 
the value of its divisions as compared to the image of an 
object of known size. For this purpose it is simply nec- 
essary to place a stage micrometer under the microscope 
and focus upon it with the ocular micrometer in position. 
The ratio of ocular divisions to stage divisions (or rather 
to the images of stage divisions) can then be read off 
and, knowing the actual value of the stage divisions, it is 
easy to calculate the number of microns on the stage 
which correspond to one ocular division. When once 
the ocular micrometer is standardized for a given system 
