248 THE USE OF THE MICROSCOPE 



3. Total Reflection Prism. — Replace the plane mirror 

 of the microscope by an unsilvered right-angled reflecting 

 prism, with equal transmission faces, about 25 or 30 

 millimeters square. Slant the microscope about 45 degrees, 

 and note with the 10 objective that the image of a 3- 

 millimeter diaphragm on the source of light is as bright as, 

 or brighter than, that given by the plane mirror, and is 

 single. Gradually raise the source of light, or set the 

 microscope axis more upright, or both, until a point is 

 reached where the reflection is much dulled. This marks 

 the end of total reflection. Note that, to correct this, 

 the source of light must be lowered, or the microscope 

 more inclined. If the reflecting side of the prism were 

 silvered, this loss of light would not occur. (Dallinger.) 



4. Large Source of Light for Uncorrected Condenser. — 

 Use an uncorrected condenser, dry or water immersed. 

 Take a smear preparation, such as bacteria of the mouth 

 stained with gentian-violet, for object. Use a 100 fluorite 

 (or 90 apochromatic) oil-immersion objective of 1.3 aper- 

 ture, and a slide and cover of standard thickness. Have 

 a ground-glass disc, 2 or 3 inches across, close to the lamp, 

 and 10 inches from the condenser. Use Wratten screen 

 No. 66. Get the largest possible light circle on the back 

 of the objective (and also, of course, at the eyepoint) by 

 focusing the condenser. Change to the 3-millimeter 

 diaphragm on the source of light. Note the reduced size 

 of the condenser light circle on the back of the objective, 

 the marginal ring, the loss of light, and the effect on the 

 resolution and definition of the object. Try to get the 

 largest solid cone by both enlarging the source and opening 

 or closing the condenser iris.^ 



5. Marginal Rays for Fine Resolution. — VJ&e an unad- 

 justed achromatic condenser; such as one corrected for 

 parallel rays (cloud light) used with a near lamp, or one 

 corrected for oil-immersion used with water-immersion, 

 or one corrected for a 1.3-millimeter slide used with a 

 1 -millimeter slide. These condensers will show different 



^ See Watson's Microscope Record, No. 16, pp. 26-28. 



