38 THE ELEMENTS OF VEGETABLE HISTOLOGY 



removed after use. This is best done by using soft 

 lens paper moistened with xylol. 



The working distance of an objective is the space 

 which intervenes between the specimen and the end 

 lens of the objective when the latter is properly focused 

 upon the object. The working distance is in inverse 

 ratio to the magnification, lower-power objectives 

 having a greater working distance than those of higher 

 power. Many objectives bear figures indicating their 

 power to distinctly show cellular structures separated 

 from each other by very small distances. This prop- 

 erty is termed resolving power and is expressed in 

 terms of numerical aperture (N. A.). The resolving 

 power of a lens is in direct ratio to its numerical 

 aperture; therefore an objective of N. A. 1.30 will 

 be of greater use in the examination of very fine details 

 than an objective of N. A. 0.85. 



Oculars. — As previously stated, the function of an 

 ocular is to further magnify the magnified image pro- 

 duced by the objective. The usual type of ocular 

 (Huygenian or negative), consists of two plano-convex 

 lenses separated from each other by considerable 

 distance. A diaphragm or metal plate with a cir- 

 cular opening is placed between the lenses of the ocular. 

 The magnification of an ocular is indicated by mark- 

 ings on the top plate or on the tube. In the Con- 

 tinental system, magnification of oculars is empir- 

 ically indicated by numerals ranging from No. 1, 

 the lowest, to No. 6, the highest. In the English 

 system, degree of magnification is indicated in inches 

 or fractions of an inch ranging, from 3 inch, the lowest, 

 to i inch, the highest. These figures have a fixed 

 relation to the equivalent focal distance as explained 



