54 ELEMENTS OF APPLIED MICROSCOPY. 
of lenses and length of draw-tube, the stage micrometer 
may be discarded. 
In using the ocular micrometer it is necessary, as we 
have seen, that the diaphragm in the eyepiece should be 
at a proper level or the scale will not be distinct. Too 
strong a light also blurs the image of the micrometer 
divisions. The object to be measured and the scale 
should be arranged in relation to each other, by rotating 
the eyepiece and adjusting the slide, so that one edge of 
the object shall coincide with the edge of one of the lines 
of the scale. Since these lines have a considerable thick- 
ness, care must be taken to refer the object to corre- 
sponding edges of the lines at its extremities. The diffi- 
culty of estimating the boundaries of objects with accu- 
racy Is always great; and the limits of precision of meas- 
urement in the best microscopic work is about .2 p. 
3. Measurement with the Camera Lucida.— Any 
method by which the image of an object may be com- 
pared with the image of a scale of known dimensions will 
serve for measurement with the microscope; and, although 
the ocular micrometer is the simplest and most satisfac- 
tory process, there are several others, of which one involv- 
ing the use of the camera lucida may be briefly men- 
tioned. This apparatus, whose external appearance is 
shown in Fig. 26, fits over the eyepiece of the microscope. 
It contains a system of mirrors which reflect the rays of 
light coming from a paper laid beside the microscope, so 
that they enter the eye along with the rays which pass 
up through the microscope. Thus the image of the 
object upon the stage and that of the paper are super- 
