90 THE USE OF THE MICROSCOPE 



For correcting a condenser, made for daylight, for 

 distance of lamp (and, at the same time, for water immer- 

 sion instead of oil), we may use small telescope objectives, 

 or the projection objectives made by some makers, if 

 of correct focal length. These range up to 30 milli- 

 meters across, which seems large enough for any condenser. 

 They can be centered in the diaphragm carrier, below the 

 condenser; or half of a photographic (rapid rectilinear) 

 objective of correct focus, or a large doublet achromatic 

 magnifier may be employed. If the condenser is primarily 

 adjusted to plane waves, as is the aplanatic-achromatic 

 condenser of one or more makers, the focal length of the 

 additional lens should be equal to the lamp distance, with 

 oil immersion of the condenser; and should be less than this 

 (depending on slide thickness) if water immersion of the 

 condenser is to be used instead of oil immersion. With 

 a slide thickness of 1.0 millimeter, the writer finds an 

 additional achromatic converging lens of about 13-centi- 

 meter focus will work well, for a condenser of 8.5-millimeter 

 focal length. The ring test will show which side of the 

 additional lens should be put in front, and what lamp 

 distance is best. 



The Aperture Circle of the Condenser. — The distribution 

 of light on the back of the focused objective is worth 

 examining regularly when using a condenser. The same 

 distribution can be seen in the circles over the eyepiece, 

 when magnified 10 or more times by a hand lens, or by a 

 compensating eyepiece of 15 magnification. 



When a properly adjusted condenser is focused on the 

 object, with a small source of light, there is seen on the back 

 of the high-power objective, as already stated, a circle of 

 uniform light, equal to the used aperture of the condenser 

 (Fig. 19). This circle may be cut down to any extent by 

 the iris of the condenser. If the condenser is now raised, 

 some of this light may go to the margin (because of under- 

 correction of spherical aberration), when only a small 

 disc will remain in the center, with a dark ring between. 

 In this focus of the condenser, the middle zone of the 



