86 THE USE OF THE MICROSCOPE 



A condenser corrected for rays away from the axis, accord- 

 ing to the sine law, can alone be called truly aplanatic 

 (Abbe). Hence the best condensers are achromatic-apla- 

 natic (Fig. 18). Of course, no useful condenser can be as 

 well corrected as an objective (Nelson), and none needs 

 to be. A well-corrected condenser can only give the best 

 results if all the surfaces between the source of light 

 and the object are corrected to correspond. These adjust- 

 ments include the distance of the source of light, the 

 parallel-plane condition of the yellow-green or other light 

 filter, the correctness of the plane mirror or reflecting 

 prism, the thickness of the shde, and the nature of the 

 immersion fluid below the slide. 



Dry and Immersion Condensers. — It is usually found, 

 in the writer's experience, that the use of cedar oil on an 

 immersion condenser is abandoned after some days, weeks, 

 or months of constant use; because of its soiling the slides, 

 the stage, and the condenser. If the immersion condenser 

 is henceforward used dry, it would have been better to have 

 procured a dry condenser at first. Even Coles (46) mostly 

 used a dry condenser; though, of course, aware of the 

 superior advantages of an immersion one. A dry achro- 

 matic condenser is susceptible to different thicknesses 

 of slides; but if slides are used of the same thickness, 

 preferably perhaps 1 millimeter, and the condenser is 

 corrected to this thickness and to the distance of the lamp 

 (if it is originally corrected for plane waves) by adding a 

 corrected converging lens in front, then a good achromatic 

 dry condenser may give an aperture of 0.8 to nearly 1.0 

 for the center of the field. It must not be forgotten that 

 Abbe himself long ago calculated an achromatic condenser 

 (15). The writer has used a dry Abbe achromatic condenser 

 with large lenses, corrected for a near lamp, for years; 

 and it permits of the use of fairly small sources of light. 

 A dry achromatic condenser, corrected for a near lamp, 

 and used with uniform slides, is the form which might 

 well, in the writer's opinion, be used wherever possible for 

 the routine work of a laboratory where oil-immersion 



