274 THE USE OF THE MICROSCOPE 



125. When are the images given by an oil-inniiersion objective much 

 impaired? 



126. What is the chief use of a water-immersion objective? 



127. Why should a water-immersion objective of high power have a 

 correction collar? 



128. How are the thicknesses of the cover-glasses to be measured? 



129. What precautions are to be taken with a high-power water- 

 immersion objective? 



130. What special advantage has the water-immersion objective in 

 searching slides? 



131. Why is the oil-immersion objective to be preferred for objects in 

 balsam? 



132. Why is it stated that the concave mirror should be abandoned on 

 the high-power microscope? What would do its work? 



133. How is a reflecting prism superior to a plane mirror? 



134. How is a sliding bar on a square stage an advantage, if no screw 

 object traverser is fitted? 



135. Why should the places of the objectives be marked on a revolving 

 nosepiece? 



136. When can a centering condenser sleeve be dispensed with? 



137. What are two uses of closely centering the fields of the low- 

 power objectives with the image field of the high objective? 



138. What are the usual boundaries of the mechanical tube length? 



139. How can an 11-millimeter objective of 0.35 aperture be adjusted 

 for use without a cover-glass? 



140. What amount of adjustment is required by a 40 objectiv^e of 0.85 

 N. A., for different cover thicknesses? (N. A. = numerical aperture.) 



141. What is an amplifier? 



142. What is the homal? 



143. What substitutes can be advantageously employed for the 

 high-power Huyghenian eyepieces (with eyepoint near the top lens) 

 (a) with low-power achromatic objectives, (6) with high-power achro- 

 matic objectives? 



144. What are compensating eyepieces? 



145. Why is the eyelens of a Huyghenian eyepiece smaller than that 

 of a corresponding compensating eyepiece without a field lens? 



146. Why should eyepieces be parfocal? 



147. What is the eyepiece constant? 



148. Give an accurate method of measuring with the microscope. 



149. How can the magnification of an objective be measured? 



150. Why is the Greenough binocular to be regarded as the most 

 important microscope for low-power work? 



151. Criticize the writer's suggestions for standard microscopes. 



152. Why is the non-stereoscopic binocular most suitable for the 

 routine work of a laboratory? 



