90 THE USE OF THE MICROSCOPE 



on the wlide, without j^round jilass. When the bright field 

 condenser is used iirst to center tlie light, dense screens 

 (or ground glass) must usually be interposed. 



The dark-field condenser is well worth using in many 

 cases, since it usually gives full resolution, equal to a full 

 well-corrected condenser cone with the bright field; rests 

 the eye; brings out slight differences of refraction (as in 

 living bacteria) ; gives good pictures of lightly stained 

 objects; and shows up any errors of adjustment of the 

 objective or of the tube length conspicuously. Its chief 

 disadvantages are that it cannot well be used for objects 

 in layers of water thicker than a few microns ; that it takes 

 time to adjust; that it does not show true colors; and that 

 it is useful chiefly for submicroscopic, or nearly submicro- 

 scopic, isolated objects. 



Summary. — There are four usual kinds of bright-field 

 condensers: 



1. The two-lens uncorrected condenser, used dry, with 

 a large source, or with a disc of ground glass close below 

 the condenser iris. This condenser causes loss of aperture, 

 and allows glare. 



2. The achromatic condenser, for use dry, corrected for 

 a slide 1.0 millimeter thick and for a lamp distance of 

 25 centimeters. The aperture should be nearly 1.0, with 

 about 12 millimeters focal length. This condenser is good 

 for the routine microscope. 



3. The aplanatic aspheric condenser is also to be adjusted 

 for a sHde of 1.0 milhmeter and a lamp distance of 25 

 centimeters; but should be corrected for water immersion, 

 and used only with yellow-green glass. This condenser is 

 suitable for achromatic objectives. Its aperture is about 

 1.3. 



4. The achromatic aplanatic condenser, adjusted as 

 No. 3, and also used as water immersion, is of about 1.3 

 aperture. This condenser may well be used with fluorite 

 and apochromatic objectives. It gives somewhat better 

 results than No. 3, especially with white or hght yellow- 

 green illumination, and is easier to focus correctly, since the 

 colors focus together. 



