silicious case is not of a smooth unbroken texture : 

 it is, in fact, marked on each side of the middle 

 rib, over its entire face, with minute striae, straight 

 cross-hatched lines having the aspect of an ex- 

 quisitely fine screen. 



Yet this is not all. This screen-like sculpturing 

 is of a kind that is deserving of still further scru- 

 tiny. And for that purpose we are now obliged to 

 resort to that last word in the optics of microscopy, 

 the oil-immersion objective lens. 



In reference to the use of this highly technical 

 tool, a brief preliminary explanation should be 

 made. 



The modern microscope, like the modern motor 

 of the automobile or airplane, because of its many 

 mechanical appurtenances and other external fea- 

 tures for refining its operation, appears to the 

 uninformed as a complex and imposing thing. 

 Fundamentally, however, it is simplicity itself. 

 Nor are the principles involved in the theory and 

 practice of its optics less simple. Reduced to its 

 barest parts, the microscope consists merely of a 

 tube containing two lens systems — one system at 

 the upper end of the tube, called the eyepiece ; one 

 system at the lower end, the objective. As it is the 



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