96 MICROSCOPIC ILLUSTRATIONS. 



Whenever high powers are used, a safety slider- 

 holder, similar to that shewn by figs. 25 and 26 in 

 the plate, or fig. 20, page 106, should be chosen for 

 holding the object slide. 



In the case of Engiscopes, or Compound Microscopes, the 

 arrangement for making observations will be different. 

 First, the triangular bar is to be raised, and the bent 

 screwed arm brought over to the centre of the stage. 

 Then screw into it the body or tube. To the upper extre- 

 mity fit an eye- piece, of which, in a complete microscope, 

 there are several of different powers : the longest is 

 always the lowest power, and is marked A. Its angular 

 aperture, which determines the size of the field of view, is 

 generally less than that of the others (if constructed on the 

 Huyghean plan), being limited by the diameter of the 

 body. It is usually a little under 30 degrees. The next 

 eye-piece or middle power, marked B> and the deepest, 

 C, have more than 30 degrees of angular aperture. Their 

 amplifying powers are commonly in the proportions given 

 given below. It is not unusual, however, to have eye- 

 pieces of intermediate powers, to suit any particular 

 class of objects we desire to investigate. 



Proportional Powers of Eye-pieces. 



Names. Diameters. Surfaces. 



A 1 ^ ; v r/ - 1 



B - - - - 2 -.,,-. VI -.,'..- 4 



C - ... - , r, .- 4 to 6 -^jjj - 16 to 36 



For viewing thin sections of recent or fossil woods, 



