THE CONDENSER RO 



3. ^^'itll a siiuill (liaplu'agni on the .source ofliglil, (he cou- 

 ckuiscr Is moved up aiul down, while the back of the objeeti\'c 

 is observed; a high-power oil-immersion objective being used, 

 and a nine-tenths condenser cone. If a marginal ring of light 

 comes by moving the condenser up, the condenser is 

 undercorrected; and, which does not so often happen, 

 if the marginal ring is focused by moving the condenser 

 down, it is overcorrected. (In some condensers only one 

 zone out of three is correct. These are usually adjusted 

 for the middle zone.) A well-corrected and well-adjusted 

 condenser shows no marginal ring (Fig. 19). This is 

 probably the best method of testing a good condenser 

 rapidly. 



Use of the Condenser. — The employment of the con- 

 denser comprises: (1) adjusting, (2) focusing, (3) regulating 

 the upper limit of the aperture by the iris diaphragm, and 

 (4) when used for dark field, regulating the lower limit of 

 the aperture by the size of the central stop. The adjusting 

 of a condenser includes: (1) the adjustment of the distance 

 of the lamp; (2) the addition of an achromatic converging 

 doublet, or unscrewing of the top lens or lenses, to change 

 from parallel to diverging incident rays, or to change from 

 oil immersion to water immersion; and (3) adjusting for 

 thickness of slide. The different adjustments are given 

 in the adjoined table. They can all be made, once for 

 all; if uniform slides, 1.0 millimeter thick, are regularly 

 used. 



Causes of Overcorrection Causes of Undercorrection 



1. Moving the lamp farther away. Moving the lamp nearer. 



2. Adding a converging lens close Adding a diverging lens close below 



below the condenser iris. the condenser iris. 



3. Unscrewing the top lens. Moving the lenses closer (if possible). 



4. Making immersion fluid of con- Using a less refractive immersion 



denser more refractive. fluid. 



5. Increasing the thickness of slide. Decreasing the slide tliickness. 



Thus the condenser can be adjusted and tested, by the 

 ring test mentioned above, until it gives the highest solid 

 aperture with a small (3-millimeter) source of light. 



