154 Animal Micrology 



eter scale, the magnification as well as the real size of the object is readily 

 calculated. 



The size of the image projected onto a piece of drawing paper at the 

 level of the table, however, does not represent the true magnifying power 

 of the microscope. The latter is really considerably smaller if the micro- 

 scope is in a vertical position because the magnification of a lens or a 

 system of lenses is calculated in terms of the conventional distance of 

 vision (250 mm., see page 144) while the distance from the ocular to the 

 table is considerably more than 250 mm. Since the rays of light diverge 



FIG. 69. Filar Micrometer. 



after leaving the ocular, manifestly, the projected image will be larger at 

 the level of the table than at a level just 250 mm. from the point of emer- 

 gence of the rays from the ocular. To determine the actual magnifica- 

 tion of the microscope, therefore, one would have to bring the drawing 

 surface to within 250 mm. of this point of emergence, sketch the pro- 

 jected scale of the stage micrometer on the paper, and then, by means of 

 an ordinary metric rule, compute the number of times the divisions of 

 the micrometer scale have been magnified. The standard distance of 

 250 mm., if the Abbe camera lucida is used (with camera mirror at 45), 

 includes the distance along the mirror-bar from the optical axis of the 

 ocular to the mirror, plus the distance from the mirror to the drawing 

 surface. 



In practical work it is not necessary to make drawings or measure- 

 ments exactly at this standard distance; one needs only to have a scale 

 made out for the distance from the camera lucida at which the drawings 

 are actually to be made, although it must be carefully borne in mind 

 that any variation in the elevation of the drawing surface will alter the 

 size of the projected image. A series of carefully prepared scales for 



