ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



■03 



The available light is derived from the end of the horizontal carbon 

 and falls directly into condenser number one, then passes through con- 

 denser number two, both of which are in the conden ser-cell attached to 

 the front of the plate. The light next passes through the water-tank, 

 then through condenser number three, which is at tached to the tank, 

 then passes to the substage condenser if high-power objectives are being 

 used, next through the object mounted on the stage of the Microscope, 

 then through the objective, and, lastly, through th e amplifier and falls 

 upon the screen. The substage condenser is not used with low-power 

 objectives, as it produces a cone of rays with too wide an angle. The 

 amplifier is a single plano-concave lens of 5 or 6 in. focal length and 

 may be used with any objective to increase the magnification of the 



Fig. 133. 



image on the screen. It is preferable to a regular microscopic ocular, 

 as it intercepts less light. 



In working with high-power objectives it is necessary to bear in mind 

 that the field is sometimes only one-fiftieth of an inch in diameter, and 

 that perfectly uniform illumination is essential for successful work. By 

 observing an arc through smoked glass, or through a combination of two 

 plates of deep ruby and blue glasses, it is readily seen that the arc 

 creeps about on the ends of the carbons as they are slowly consumed. 

 The luminous point consequently varies in position. This difficulty 

 may be overcome in three ways : — (1) By using soft-cored carbons ; 

 (2) by using as small a horizontal carbon as possible in combination 

 with a larger vertical carbon ; (3) by moving the luminous point into 

 the optical axis by a slight vertical or lateral movement of the arc. To 

 accomplish the last, the vertical supporting rod of the lamp should not 



