PRACTICAL MICROSCOPY. 



traded somewhat, producing corrugations or folds, and 

 consequently the rays proceed from different planes, so 

 that the test should be to see how much or how little of 

 the total depth can be seen under one focussing, without 

 indistinctness. 



As an instance of the work specially suited for low 

 angles and consequent penetration, the cyclosis in Vallis- 

 neria spiralis is often cited. The author can assure his 

 readers that this may be easily seen with advantage under 

 a J-inch objective of 80 air angle (0*64 numerical aper- 

 ture), the largest yet made for this power in this country. 



5. RESOLVING POWER. Without entering into any of 

 the theories of this property in objectives, it may be briefly 



