CHAPTER XXVIP 

 FIFTY PRACTICAL EXERCISES WITH THE MICROSCOPE 



1. Test of the Ground Glass. — Test the ground glass 

 for the passage of other than diffracted (or refracted) 

 Hght by trying to form an image, on a screen before the 

 microscope, of the incandescent filament; having before 

 the screen a large condensing lens between once and twice 

 its focal length from the lamp. It is best to form the image 

 first, and then insert the ground glass. Only rays passing 

 through minute unground portions of the glass would help 

 to form such an image. 



2. Glare and Large Source of Light. — Take a 40 objec- 

 tive of 0.85 aperture, or the 70 water-immersion objective 

 of 1.25 aperture (with correction collar). Focus the 

 objective on a well-known object with fine but sharply 

 stained details, such as delicate bacteria or spirochetes, 

 or fine chromosome threads with chromomeres at the 

 leptotene stage. Use a cover-glass 0.16 to 0.17 millimeter 

 thick; or adjust the objective for cover-glass thickness by 

 altering either the tube length or the correction collar. 

 With a fully corrected immersion condenser, get a ^i cone 

 with a large source of light, such as a 5-centimeter ground- 

 glass disc at 25 centimeters distance. Note the glare 

 obliterating details on attempting to increase the aperture 

 of the condenser light circle to seven-eighths or nine-tenths 

 of that of the objective. Put a 5- or a 3-millimeter dia- 

 phragm close to the ground glass, and focus it. Note that 

 the glare disappears, and that the aperture can now be 

 increased to % or %o- (The intensity of the light must 

 be properly adjusted by screens.) Try the same with an 

 oil-immersion objective. (Beck, Nelson, Hartridge.) 



^ Some few of these exercises have not been tested by the writer. They 

 are then of the nature of suggested experiments. 



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