PRACTICAL EXERCISES WITH THE MICROSCOPE 253 



needed, since it has only to correct for the undercorrection 

 caused by the use of water instead of oil, and a thinner 

 slide. Half of a rapid rectiUnear photographic lens of the 

 correct focal length and diameter may be sometimes used to 

 advantage. 



15. Aplanatic Aspheric Condenser and Green Light. — 

 An aspheric condenser is well corrected for spherical 

 aberration, but not corrected at all for color. It may, or 

 may not, be corrected for plane waves. An aspheric 

 condenser (made by Bausch and Lomb) has a focal length 

 of 12 milUmeters and requires a somewhat weaker accessory 

 lens to correct it for lamp distance and water immersion 

 than the achromatic condenser mentioned in 14. Note 

 that it has different foci for different colors ; using the image 

 of the diaphragm on the source of hght, with an oil-immer- 

 sion objective. Employ Wratten screens Nos. 66, 56, 57A, 

 58, and 61 in succession, and note the diminution of the 

 range of color focus. With the denser screens, this con- 

 denser, when properly adjusted for lamp distance and 

 water immersion, may be nearly equal to the achromatic 

 condenser referred to in 14, but without yellow-green 

 screens it is inferior. This may be tested. 



16. Dark Field with Immersion Achromatic Condenser. — 

 Using a well-corrected and adjusted water-immersed 

 achromatic and aplanatic condenser, with accessory correct- 

 ing lens; center two star discs, accurately cut out of some 

 thin metal or card, close under the lower lens or iris of the 

 condenser. They may cut out condenser apertures of 

 0.75 and 0.95, respectively, for objectives with apertures of 

 0.65 and 0.85. Since their diameters are proportional to 

 the apertures they cut off, the diameters can be calculated 

 from the maximum useful diameter of the condenser lens 

 or iris next them, and the known maximum aperture of the 

 condenser. For a source of light, a 6-volt, 108-watt, 

 tungsten-ribbon lamp may be employed. The ribbon is 

 focused directly on the sHde. The 20 apochromatic 

 objective, with a high eyepiece, gives good images, the cover 

 being 0.17 millimeter thick, or slightly less for objects in 



