THE CONDENSER 87 



objectives are employed. The focus need not be altered 

 for different slides, in ordinary work, if only 1.0-millimeter 

 slides are used. For high-power research, water-immersion 

 (or oil-immersion) condensers should, it seems, nearly 

 always be used. It was the custom, when daylight was 

 the popular source of light, to correct condensers for 

 parallel rays (plane waves). This is now unnecessary; 

 for since electric lamps are used for all high-power work, 

 such condensers have to be corrected again by an additional 

 achromatic lens below, which would not be needed if 

 they were made for a lamp at 25 centimeters. Also even 

 dry condensers are often, it seems, corrected for a thick 

 slide, instead of for a slide between 0.9 and 1.1 miUimeter. 

 It would be a boon if some optical firm would make a 

 condenser (achromatic and aplanatic), of 1.25 true aperture 

 with a 3-millimeter source of light, water-immersion, 

 with lower lens about 30 millimeters across; and corrected 

 for a lamp at about 25 centimeters, and for slides 1.0 

 millimeter thick. The upper surface of the upper lens 

 might well be concave, and so help to hold the water when 

 the microscope is slanted (as well as improve the correction) . 



In the writer's experience, the upper lens of a water- 

 immersion condenser, as well as the lower surface of the 

 slides, has to be regularly cleaned with xylol, since traces of 

 immersion oil get on them and diminish the light 

 perceptibly. 



Testing the Adjustment of the Condenser. — There are 

 three ready methods of testing the condenser as to its 

 adjustment and aperture. Some such tests are imperative 

 if the best images are to be formed. A condenser out of 

 adjustment gives a lower aperture, which is especially 

 noticeable with a small source of light. 



1. The source of light being properly diaphragmed, 

 various objectives of known aperture are focused on an 

 object, the object being in water if a water-immersion 

 objective is used, and in (balsam or) immersion oil for an oil- 

 immersion objective. The diaphragm on the source of light 

 is made of such a size as to give on the slide a focused image 



