QUESTIONS 103 



1. Paint with a earners-hair brush, around the concavity of a 

 concave slide, a ring of vaselin. 



2. Clean cover-glass particularly well, so that it is free from 

 grease; hold in a pair of forceps, and with a platinum loop, place on 

 the centre of the glass a small drop of water, or, better, sterile 

 physiologic salt solution. 



3. Enter culture tube with sterile platinum loop and remove 

 from the surface of the agar, gelatin, etc., a small bit of the growth, 

 avoiding at the same time to take any of the culture medium. 



4. Rub up the small bit of growth with the drop of water on 

 the cover-glass, so that there is formed a uniform emulsion of the 

 bacteria in water. Spread the drop out considerably so that it is as 

 shallow as possible. 



5. As soon as this is accomplished, place the cover-glass over the 

 concavity of the concave slide in such manner that the drop hangs 

 down free into the hollow space. 



6. Now press cover-glass down gently into the vaselin surround- 

 ing the concavity, so that the latter is closed air-tight. 



7. The hanging drop is now ready to be examined in the manner 

 described above. 



When bacteria in a fluid excretion like urine, or from a fluid culture 

 medium like nutrient bouillon, are to be examined a drop of these 

 fluids can be placed directly upon the cover-glass without first apply- 

 ing a drop of water. 



QUESTIONS. 



1. What is meant by a pure culture of a bacterium? 



2. What is a colony of a pure culture? 



3. At what time in the development of a bacterial growth can colonies best 

 be studied? 



4. What properties of bacteria in pure cultures can be recognized without 

 the aid of the microscope? 



5. What sources of light are employed in the use of the microscope ? 



6. What are the main parts of a modern compound microscope adapted for 

 use in work with bacteria ? 



7. Explain the terms: ocular, objective, revolver, npsepiece, draw-tube, 

 coarse adjustment, micrometer screw, Abbe condenser, iris diaphragm, plane 

 mirror, concave reflector, mechanical stage, oil-immersion lens, dry lens, focal 

 distance. 



8. For what distance are the objectives corrected? 



9. How far should the draw tube be drawn out and why? 



10. What is meant by spherical aberration of an objective? What by chrom- 

 atic aberration? 



11. What is meant by the refraction of light? 



12. How is it affected by transparent media of various densities? 



13. What is meant by the focus of an objective or its focal distance? 



14. In what relation does the focal distance stand to the magnification? 



15. Why is a high-power homogeneous immersion lens better than a dry lens 

 of the same focal distance ? 



16. Explain why very thin cover-glasses are used with the T V oil-immersion 

 objective? 



17. How is the iris diaphragm used when examining stained and unstained 

 bacteria, and why? 



18. What does the term parfocal mean? 



