THE ROUTINE MICROSCOPE 05 



by the satisfactory employment of objectives of lower aperture. 



4. The 20 achromatic objective of 0.4 or 0.5 aperture (or the 

 20 apochromatic of 0.65 aperture), used with a good condenser, 

 will do much of the work now done with the 40 objective, with 

 greater ease in manipulation. The same may be said of the 50 

 oil-immersion of 1.0 aperture, with a good condenser; as com- 

 pared with the 90 of 1.3 aperture, with an uncorrected condenser. 



5. A dry achromatic condenser, properly adjusted, will give 

 good results with an oil-immersion objective of 1.25 aperture. 

 The maximum condenser aperture will be about four-fifths of 

 that of the objective, and the maximum useful magnification will 

 be about 1125. 



6. A good electric lamp for routine work is a 100- watt C- 

 Mazda, frosted on the inside. 



7. Three one and three-quarter inch Wratten gelatin films, 

 Nos. 66, 56, and 57A, can be obtained for about a dollar. They 

 can then each be mounted in balsam between two pieces of plate 

 glass. The optical system of the achromatic microscope is much 

 improved by their use. Instead of having to partially correct for 

 a long spectrum, the existing corrections of the system have to 

 be applied only to a short piece of the spectrum, mostly yellow- 

 green, and therefore are more efficient. 



8. The use of cover-glasses 0.17 millimeter thick produces 

 crisper images, even with oil-immersion objectives. For dry 

 objectives, 0.15- or 0.16- millimeter covers are usually better, 

 because of the layer of mounting medium over the object. 



