180 
ELEMENTARY CHEMICAL MICROSCOPY 
Closely related to micrometry by direct comparison are 
measurements obtained by means of mechanical stages having 
graduated scales, as for example the types shown in Figs. 70 and 
71, pages 139, 140. The scales are usually ruled in millimeters 
and have a vernier accurate to one-tenth of a millimeter. For 
large objects whose measurements are not required with an 
accuracy greater than o.i mm. the mechanical stage will be 
found to be convenient and rapid. One edge of the object is 
brought in contact with a cross-hair of the eyepiece. The stage 
scale reading is recorded and the object moved by means of the 
milled head of the movable stage until the opposite edge is 
brought in contact with the same cross-hair, the stage scale is 
again read. The difference in the readings gives the displacement 
of the object and therefore its linear dimension in the direction 
of movement. Since both movements of the stage are graduated, 
length and breadth may be rapidly ascertained. 
Micrometry by Photography and a Projection Lantern. This 
method may also be considered as a variant of Method i. The 
measurements of many very tiny particles is very wearisome 
under the microscope as is also the search for particles of greater 
volume than a certain fixed maximum. Accurate focusing upon 
material of variable size is difficult and annoying. A satisfactory 
substitute consists in using a moderate power objective and 
photographing the preparation. The negative may be placed 
in a projection lantern and the image thrown on a screen. The 
images of the particles are now greatly enlarged and may be 
measured with an ordinary millimeter rule. Knowing the mag¬ 
nification of the image, the actual size of the particles may be 
readily computed. The magnification is determined by photo¬ 
graphing a stage micrometer under' exactly similar conditions 
and projecting the photograph on the screen. This being done 
once for all, future measurements become quite simple. In 
routine work this procedure will be found more rapid and less 
tiresome than the other methods described. 
Method 2. — Measurements obtained by means of a stage microm¬ 
eter and camera lucida. Lay the object upon the stage under the 
microscope, over the ocular of which some form of drawing 
