30 THE MICROSCOPE, ETC. 



Rarities of microscopic objects, it follows, that their 

 accurate delineation depends as much on the cor- 

 rections of the condenser through which the light 

 passes to the object, for achromatism, and on the size 

 of its angular aperture, as it does on the perfection 

 of the objective in these particulars. 



It must be remembered however that a condenser 

 is not necessary when using the one inch or lower 

 powers, but is indispensable with those above it. 



A good achromatic object glass of the French form, 

 consisting of three powers, screwing on top of each 

 other will answer the purpose, if the microscope is 

 not already fitted with Abbe's illuminator or one of 

 Powell and Leland's condensers. When in use it 

 should be fitted in the centre of the ring A so as to 

 be removable at pleasure, and with the face of the 

 uppermost combination flush with the top of the 

 substage tube, to allow of its close approximation with 

 the under surface of the glass slip on which the 

 object is mounted. 



Besides the condenser itself being freely remov- 

 able from the ring in the sub-stage, the three com- 

 binations which compose it should be easily un- 

 screwed from one another, as on no account must the 

 power of the condenser exceed that of the objective ; 

 we therefore require no condenser with the one inch, 

 while with the quarter two and with the eighth all 

 three combinations are necessary. 



If the condenser is not an immersion, a pin hole 

 cap similar to that figured in section 4, fig. 16, will 

 be found an extremely useful addition placed on the 



