COMPOUND MICROSCOPES 11 
2. The Abbé condenser (Fig. 9) is a combination of two or 
more lenses, arranged so as to concentrate the light on the 
specimen placed on the stage. The condenser is located in the 
opening of the stage, and its uppermost 
surface is circular and flat. 
3. Objectives (Figs. 10, 11, and 12). 
There arelow, medium, and high-power 
objectives. The low-power objectives have 
fewer and larger lenses, and they magnify 
least, but they show more of the object than 
do the high-power objectives. Pies de =k bbe 
There are three chief types of objec- @andlenser 
tives: First, dry objectives; second, wet 
objectives, of which there are the water-immersion objec- 
tives; and third, the oil-immersion objectives. The dry 
objectives are used for most histological and pharmacog- 
nostical work. For studying smaller objects the water ob- 
BiGs 11: 
Objectives. 
jective is sometimes desirable, but in bacteriological work the 
oil-immersion objective is almost exclusively used. The globule 
of water or oil, as the case may be, increases the amount of light 
entering the objective, because the oil and water bend many 
rays into the objective which would otherwise escape. 
4. Eye-pieces (Figs. 13, 14, and 15) are of variable length, 
but structurally they are somewhat similar. The eye-piece 
consists of a metal tube with a blackened inner tube. In the 
