SECTION OF ECHINUS SPINE. 113 



one focussing, the margin as well as the centre. For 

 higher powers similar objects may be used, but of course 

 of smaller dimensions, while freedom from distortion can 



FIG. 107. 



be tested for, by observation of the micrometer placed upon 

 the stage, or a series of lines ruled by Wheeler, varying 

 from one thousand to ten thousand to the inch. 



4. PENETRATING POWER. " Penetration," as it is 

 understood now, signifies that property which an objec- 

 tive possesses whereby several planes of an object are 



I 



