272 THE USE OF THE MICROSCOPE 



76. Why should the glass of the light filters be nearly plane ijarallel? 



76. What are the advantages of yellow-green light over daylight (or 

 tungsten light altered to daylight value)? 



77. How may a graduated series of Wratten yellow-green light filters 

 ])erform two functions? 



78. Name five ways of regulating the intensity (not the aperture) of 

 the light. 



79. For what are screens of other colors (not yellow-green) sometimes 

 useful? 



80. Why are achromatic objectives so nmch improved by yellow- 

 green screens? 



81. What are the disadvantages of the uncorrected (two-lens) Abbe 

 condenser? 



82. How can the aspheric (aplanatic) condenser be correctly used? 



83. What is probably the best condenser for routine laboratory work? 



84. Why is water immersion to be preferred for the immersion 

 condenser? 



85. Why is the achromatic-aplanatic condenser to be preferred? 



86. How can a condenser be tested as to adjustment for shde thickness 

 and lamp distance? 



87. How can an oil-immersion condenser be corrected for water 

 immersion? 



88. How can the aperture of the light cone delivered by the condenser 

 to the object be estimated? 



89. What happens to the aperture of the light cone when the con- 

 denser is out of focus? 



90. Why must not the iris of the condenser be used for regulating the 

 intensity of light? 



91. What adjustments are most necessary when a % or %q cone of 

 light is to be used? 



92. W^hat happens if the condenser delivers a larger aperture than 

 that of the objective? 



93. How can an immersion achromatic condenser be arranged to show 

 small spirochsetes distinctly? 



94. Why is the large bicentric reflecting condenser usually preferable 

 to the small pattern? 



95. What are the best kinds of objects for the bicentric and cardioid 

 condensers? 



96. Why are special oil-immersion objectives necessary for these dark- 

 field condensers? 



97. How should the slides and covers be adjusted for best vision with 

 reflecting condensers and objects in water? 



98. For what objects are the dark- field condensers, used with objec- 

 tives of 1.3 aperture, useful? 



