PRACTICAL EXERCISES WITH THE MICROSCOPE 265 



arc known. Use llii.s cyci)i(H;o scale in tlic niici-uscopc 

 with an object micrometer scale on the stage. The 

 comparison of the two gives the magnifying power (new 

 style) of the objective directly. If the divisions of the 

 eyepiece scale are 0.1 millimeter, and those of the object 

 scale are 0.01 millimeter; then the magnifying power of the 

 objective is equal to the number of eyepiece divisions con- 

 tained in 10 divisions of the magnified object scale. 



41. Use of Light Screens. — With most objectives, the 

 light intensity to be regularly employed should be arranged 

 so as to be too high on using maximum aperture (at maxi- 

 mum useful magnification). This extra intensity permits 

 the use of blue or green screens. Take a 60 apochromatic 

 objective of 1.4 aperture, used with a 100-watt, low- 

 voltage lamp and ground glass. Have a corrected immer- 

 sion condenser, cover-glass and slide of standard thickness, 

 and a 3-miUimeter diaphragm on the source of light. The 

 object may be a well-stained preparation mounted in 

 immersion oil, and showing fine details, such as the chro- 

 momeres of Lilium at the pachytene stage. Open the con- 

 denser iris to a maximum aperture of about 1.3. Note 

 that the light is too intense for optimum images, even 

 with a 20 eyepiece on the binocular. Try different screens 

 of the Wratten series, Nos. 66, 56, 57A, 58 or 61, or two 

 combined, until the hght is reduced in intensity enough 

 for maximum aperture to be allowable. (Coles, Metzner.) 



42. Swan Cube as Vertical Illuminator. — Try the vertical 

 illuminator with a small semisilvered cube (like that in the 

 monobjective binocular, but smaller) over the shortened 

 oil-immersion objective corrected for use without a cover- 

 glass. Have the three unused sides of the cube blackened 

 with carbon mixed with balsam. Try if the illumination is 

 superior to that given by the usual parallel-plane glass 

 plate, which also gives full aperture. (The Spencer Lens 

 Company now make such a vertical illuminator, with 

 improvements.) 



43. Metallurgical Oil-immersion Objectives for Uncov- 

 ered Objects. — Try these on the ordinary stained biological 



