30 MICROSCOPE, ETC. 



It must be remembered however that a condenser 

 is not necessary when using the one inch or lower 

 power, but is indispensable with those above it. 



A good achromatic condenser of the French form, 

 consisting of three powers, screwing on top of each 

 other will answer the purpose. 



A much better form is Powell and Leland's new 

 homogeneous immersions; whichever is used, it should 

 be fitted in the centre of the ring A so as to be remov- 

 able at pleasure, and with the face of the uppermost 

 combination flush with the top of the substage tube, 

 to allow of its close aproximation 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 substage, the three com- 

 binations which compose it should be easily unscrewed 

 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 eight 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 

 top combination, when using the eighth and higher 

 powers; it not only prevents the passage of any rays 

 except those actually concerned in the direct illu- 

 mination of the object and the subsequent formation 

 of the image on the focusing screen, but also if 

 brought into view with a low power objective enables 



