188 
ELEMENTARY CHEMICAL MICROSCOPY 
1878. Again revived by A. E. Wright in 1890. Thoroughly 
tested out by Ives in 1903 and independently rediscovered by 
Clendinnen in 1910.^ And yet in spite of the many times this 
principle of employing a scale of variable value as a standard 
has been independently discovered and its desirable features 
pointed out, it is almost never referred to in manuals devoted to 
microscopy. 
By means of the mirror and the Abbe condenser, it is possible 
to project into the plane of the object lying upon the stage, the 
image of a scale whose value has been ascertained. Both scale 
and object are magnified together and it therefore follows that 
no matter what may be the combination of objective and ocular 
employed, the value of the divisions of the scale image will 
remain unchanged, provided that the distance of the scale from 
the condenser is not altered. Any change in the distance of 
scale from mirror and condenser will be accompanied by a pro¬ 
portional change in the size of the divisions of the scale in the 
image projected into the plane of the object. 
In micrometry, by means of ocular micrometers, we are 
restricted to the single ocular, containing the scale, and to a fixed 
draw-tube length. To obtain a different magnification, one is 
obliged to change objectives. This means that a new ocular 
micrometer value must be employed and a record kept for every 
change in objective. Moreover, the actual sizes of the divisions 
seen in the eyepiece micrometer are constant and cannot be 
changed. 
In micrometry, by means of a scale image projected by the 
condenser, we have merely to record the distance of the scale 
from the microscope in determining its value and we may then 
adopt any possible combination of objectives, oculars or tube 
lengths, without change of value. 
A scale ruled as shown in Fig. 122 has been found satisfactory 
by the author and has been in general use in his laboratory for 
a number of years. This scale, a photographic positive, is cori- 
veniently held in a vertical position by metal carriers attached 
1 See Ives, Journ. Frank. Inst., 164 , 73; Clendinnen, J. Roy. Micro. Soc., 
1910, 368. 
