THE CONDENSER 97 



Condensers made for infinite lamp distance, or for oil 

 immersion, or for both, when used as water immersion with 

 a near lamp are much undercorrected spherically. They 

 may be corrected for this, as already stated, by adding a 

 corrected converging lens below, or (partially) by separating 

 the lenses. A condenser with correction and adjustment 

 will give {a) about equal apertures with a large (25-milli- 

 meter) or a small (3-millimeter) source; (6) nearly the 

 same focus of the diaphragm on the source with the 16- 

 milhmeter and the 2-millimeter objectives; and (c), with a 

 3-millimeter diaphragm on the source, and full aperture, 

 no outer ring on the back of the high-aperture objective, 

 either within or without the focus of the condenser. 



A good dark field for objective apertures up to 0.85 can 

 be obtained by centrally diaphragming the aplanatic 

 achromatic water-immersion bright-field condenser. A 

 dark field for objects in water and objectives up to 1.05 

 (or even 1.2) aperture can be had by using the cardioid, 

 or bicentric, or other similar reflecting condenser. The 

 Cassegrain and Leuchtbild (luminous spot-ring) reflecting 

 condensers will give a good dark field for objects in immer- 

 sion oil or balsam, with objectives of apertures up to 1.3. 

 The dark field yields results equal to those of the bright 

 field with all adjustments correct. The dark field is better 

 for submicroscopic objects. Oil-immersion objectives for 

 dark field are best when made to be used at their full aper- 

 ture. Hence the special oil-immersion objectives with 

 lower aperture are necessary. 



Practical Points 



1. A corrected condenser should be used for standard micro- 

 scope work, because with an uncorrected condenser the source 

 cannot be diaphragmed to get rid of glare without simultaneously 

 decreasing the aperture. 



2. An aspheric-aplanatic condenser is considerably better 

 than an uncorrected one, but must be used with a yellow-green 

 screen, because each color lias a (liffcrent focus. 



3. An achromatic-aplanatic condenser, of aperture 1.3, 

 water immersed, is doubtless best for high-power work; especially 



