110 



PLANT ANATOMY. 



1 



Figr. 4. Sub-Stajre Condenser. 



to the advances that have been made in 

 the construction of the objectives. 



The objective is placed at the lower 

 end of the tube, either fitting directly 

 into the tube, or into the nose-piece by 

 means of what is known as the Society 

 Screw, which is an internationallv 

 adopted size, so that any objective can 

 be used in any stand. 



The objective is made up of a series 

 of lenses varying from two to four, and 

 works in a combination as a simple mag- 

 nifier; it forms a real inverted image in 

 the tube, which is further magnified by 

 the ocular. 



Objectives arc designated either arbi- 

 trarily, as a, b, c, d, e, etc., or according 

 to a system of measurements expressed 



in the English or in the 

 system. These 



in fractions 



metric 



measurements 

 signify that the magnifying power of any 

 lens, say a %-inch objective, is equal to 

 the magnifying power of a simple lens 

 whose focal distance equals ^ inch. In 



high- 



For 



front 



good 



general, therefore, the lower the frac- 

 tion which designates the lens, the 

 er is the power of magnification, 

 general purposes, **the smaller the 

 lens, the higher the power," is a 

 guide for the student to enable him to 

 know which is the high power and 

 which the low. It should also be roinem- 

 bered that, as a rule, the higher the 

 magnification, the less working distance, 

 the less field, and the less iliumination. 

 (Fig. 6.) 



J 



The Mirror is attached to the mirror 

 bar beneath the stage, and is freely 

 movable. It consists generally of two 

 surfaces, a plane and a concave, the lat- 

 ter of which is generally used; it should 

 be of ample width, at least two inches. 



The Condenser, when present, is situ- 

 ated just below the stage, and if of the 

 Abbg type, is well represented in the il- 

 lustration; it is used to give a greater 

 supply of light to the objective, and is 

 invaluable in working with stained spec- 



