THE CONDENSER 83 



soiiio adjustiiicnt is wrong so as to cause glare. Willi 

 high powers there is often glare before a nine-tenths cone is 

 reached, unless due care is taken. Details are of course lost 

 by the use of this glare method. A source of light larger 

 than the source-field gives somewhat similar results. In 

 the writer's opinion, a %q cone, with all adjustments 

 correct, gives a better view of the colors of the bacteria, 

 together with all details. In fact, in Koch's time (about 

 1878) the usual condenser cone w^as often not over 0.3; 

 which, with the oil-immersion objective of over 1.0 aperture 

 just introduced by Abbe, would cause dark lines due to 

 differences of refraction, w^hich would more or less hide 

 the colors of stained objects. Thus Koch's use of a large 

 cone on stained objects was an advance in microscopy 

 at the time. 



In the writer's opinion, the uncorrected condenser has 

 no merit over the corrected condenser, except a small 

 difference in price; and it has a number of demerits. 



The disadvantages of employing the uncorrected con- 

 denser are as follows: 



1. The nominal aperture of 1.2 or 1.4 only applies to the 

 extreme rays of this condenser when used as an oil immer- 

 sion, and the aperture is much less when the central rays 

 are focused. The condenser cannot be used with a small 

 diaphragm on the source without a large reduction in 

 aperture. 



2. Its focal length varies, being shorter for green than 

 for red; and much shorter for the marginal than for the 

 central rays. Hence, unless carefully adjusted, the wrong 

 zone or color may be focused, and thus the aperture 

 further reduced (Fig. 18). 



3. At its maximum, with a small source, the central 

 aperture, properly focused, is said (Carpenter-Dallinger) 

 to approximate, for the three-lens form, 0.5. The writer's 

 measurements make it less than this for a specimen of the 

 two-lens form. Thus, an oil-immersion objective of 1.4 

 aperture, with an uncorrected but properly adjusted con- 

 denser, used with a 3-millimeter source of light so as to 



