62 THE USE OF THE MICROSCOPE 



from 200 to 300, which is what is given by a 40 objective 

 with a 5 or 7 eyepiece. (If a 40 objective is chosen, let 

 it be the one with 0.65, not that with 0.85, aperture, for 

 the former is less sensitive to errors in cover-glass and 

 tube length.) Similarly the 50 or 60 oil-immersion objec- 

 tive, of about 1.0 aperture, can sometimes advantageously 

 replace the 90 or 100 objective, of 1.25 or 1.3 aperture. 

 As several microscopists have pointed out, the 90 objective 

 of 1.25 aperture is sometimes preferable to the 90 or 100 

 oil-immersion objective of 1.3 aperture. From the cata- 

 logues of three of the leading microscope manufacturers, 

 it appears that a dry achromatic (or aplanatic) condenser, 

 together with 10 and 20 dry, and 50 oil-immersion objec- 

 tives, costs only about $5 more than the usual set, consisting 

 of an uncorrected condenser, with dry objectives 10 and 

 40, and 90 (or 100) oil-immersion objective. Probably 

 the cheapest improvement, however, of the routine micro- 

 scope will come from the purchase of covers, 0.15 to 0.17 

 millimeter thick. 



Routine Microscope. — Certain points may well be espe- 

 cially attended to in routine microscopes. 



If daylight is used constantly, a dry achromatic con- 

 denser with lenses adjusted for plane waves will give the 

 maximum of light. (If electric light is used an additional 

 appropriate correcting lens should be added.) The writer 

 has found useful a cardboard screen over the window, with 

 a circular or elliptical aperture of variable size. This 

 lessens glare ; for the source can be made more nearly equal 

 to the source-field. A white board in sunlight (Cobb) 

 is worth setting up, for employment when suitable clouds 

 fail; for a blue sky is almost unusable with high powers 

 and color screens. 



If electric light is employed, sometimes or constantly, 

 the Corning daylight glass should be attached when 

 colors must be distinguished; or a yellow-green gelatin 

 film (Wratten Nos. 66, 56, or 57A) may be used, if exact 

 distinction of colors is not required. A good lamp foi- 

 constant use is a C-Mazda with })ulb matt internnllv, of 



