PRACTICAL EXERCISES W ITU THE MJCROSCOl'E 219 



degrees of undercorrectioii. (Soiuewhat similar, l)ut more 

 irregular results may be had with the ordinary uncorrected 

 ''Abbe" condenser, used oil or water immersed.) Have a 

 3-millimeter diaphragm as in 4, with an oil-immersion 

 objective of 1.3 aperture, and a fine well-stained object. 

 Raise the condenser, with wide iris, above the usual focus 

 for low apertures. Note the external bright ring on the 

 back of the objective. This bright annulus has an aperture 

 at the limit of the condenser, and enables the condenser 

 to be used in resolving fine diatom markings, although it is 

 undercorrected. Coarser markings, however, are not 

 simultaneously well shown. Try this raised condenser 

 on the 20 diatoms of the Moeller test plate, one by one. 

 Also try, on the same objects, annular illumination by cen- 

 tral stops before the condenser, cutting ofT different amounts 

 of the aperture of the condenser. (Conrady, Spitta.) 



6. Corrected Condenser and Small Light Source. — Take 

 a corrected immersion condenser, corrected for centering, 

 lamp distance, slide thickness, and immersion fluid; and 

 focus the source on the object. Note (with a 1.4 or a 1.3 

 aperture objective and a well-stained object in balsam or 

 immersion oil) on the back of the objective, the largest 

 cone this condenser will give; using a 5-centimeter disc of 

 ground glass close to the lamp as a source of light. Then 

 put a 2- or 3-millimeter diaphragm close to the ground 

 glass, and note the effect on the condenser light circle at 

 the back of the objective, or in the eyepiece circle of the 

 microscope over the eyelens. The better adjusted the 

 condenser, the less shrinkage of this circle will be brought 

 about by the 2- or 3-millimeter diaphragm. (Nelson, Beck, 

 Hartridge.) 



7. Focal Test for Under- or Overcorrection of Con- 

 denser. — Condensers corrected, in the old way, for plane 

 waves and for oil immersion, are of course undercorrected 

 when used with a near lamp, and either dry or water 

 immersed. Take such a condenser ; either a dry achromatic 

 condenser of 1.0 nominal aperture, or an achromatic 

 immersion condenser of 1.4 aperture when oil immersed. 



