36 THE ELEMENTS OF VEGETABLE HISTOLOGY 



2-inch objective as the lowest, and extending to the 

 tb- inch or highest power. This system is based upon 

 what is termed the equivalent focal length of the lens. 

 An objective marked i would produce a real image 

 of the same size as a simple convex lens, the principal 

 focus of which is i inch distant from the lens. The 

 metric system of marking objectives is similar to the 

 English system and differs from the latter only in that 

 magnification is expressed in millimeters of equivalent 

 focal length, and not in inches or fractions of an inch. 

 The objectives, according to this system, usually 

 range from 48 mm., the lowest, to 1.5 mm., the highest. 

 Briefly stated, the relation of marking to magnifica- 

 tion is as follows : Continental system; lower numerals 

 indicate lower powers; English and metric systems; 

 lower numerals indicate higher powers. In the case 

 of Continental objectives there is an exception to these 

 rules. As previously stated, the highest powered 

 Continental objective is ordinarily No. 9; but object- 

 ives above this power are sometimes manufactured, 

 and are marked in fractions of an inch. Thus the 

 sequence would be No. 9, tV inch, tV inch and tf inch. 

 One may judge the power of an objective by the size 

 of the end lens, as the lower power objectives have 

 larger end lenses than those of higher power. 



In lower power lenses the refraction of light rays 

 as they emerge from the upper surface of the glass 

 slide is not of great consequence. In working with 

 lenses giving high magnifications (1.9 mm., tz inch, 

 etc.), these refractions offer serious difficulty as great 

 amounts of light are lost because of the small diameter 

 of the end lens. This difficulty is overcome by placing 

 liquids between the end lens of the objective and the 



