THE CONDENSER 93 



are whitened, etc. On the other hand, if the aperture 

 of the condenser is smaller than about four-fifths of the 

 aperture of the objective, with a small light-source; then 

 there is marked loss of light, definition, resolution, and 

 brilliancy. 



It may be well, as is sometimes done, to have an arbitrary 

 scale arranged by the side of the iris of the condenser. 

 The values of this scale may be obtained by equalizing 

 the aperture of the condenser to that of standard objectives, 

 all adjustments being standardized. Such a scale should 

 be legible from the side of the condenser. 



Dark-field Condensers. — A corrected immersion con- 

 denser of long focus and 1.3 aperture, such as those made by 

 some prominent optical firms (with an additional achro- 

 matic lens to correct for lamp distance, etc.), gives a dark 

 field, when a central stop is placed near the front focal 

 plane (as already stated). An expanding central stop is 

 best (Coles). Such a dark field is preferable, according 

 to Coles, for searching for spirochsetes with an 8-millimeter 

 apochromatic objective on a dry smear preparation. This 

 combination gives also excellent pictures of living bacteria, 

 up to a magnification of about 600. The required magni- 

 fication can be fairly attained with the apochromatic 

 objective 20, and a pair of 15 eyepieces on the Siedentopf 

 binocular attachment; the eyepieces here magnifying over 

 25 times. The coil or ribbon of a 6-volt, 108-watt lamp 

 is to be focused directly on the object, using deep-green 

 screens if necessary. Yellow-green screens are as useful 

 with the dark ground as with the bright ground. The 

 cover-glass should be accurately 0.16 to 0.17 millimeter 

 thick, and should be parallel to the shde. The layer 

 between cover and slide should not be more than 10 microns 

 thick. The star test may be used to show that cover-glass 

 thickness and tube length are correct. Then almost 

 faultless images will be obtained. This form of dark 

 field gives the maximum of light (Metz, 97). Tn the 

 writer's experience, an appropriate central stop can be 

 arranged for the objective aperture of 0.85, on the aplanatic 



