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 0.1 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. 

 1 Micrometry by Photography and a Projection Lantern. This 

 method may also be considered as a variant of Method 1. 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 



