6 
PHARMACEUTICAL BOTANY 
7. The fine adjustment, a micrometer screw back of the tube, 
which, on being turned, produces a very small motion of the entire 
framework which holds the body tube. 
8. The oculars or eyepieces which slip into the upper end of the 
draw tube. Each of these consists of two plano-convex lenses, the 
lower one being the larger and known as the field lens because it 
increases the field of vision. The upper or smaller lens is called the 
eye lens. It magnifies the image formed by the objective. Midway 
between the field and eye lens is a perforated diaphragm, the object 
of which is to cut out edge rays from the image. 
According to the system adopted by the maker, oculars are desig- 
nated by numbers, as 1, 2, 3, 4, etc., or by figures which represent 
focal lengths. 
9. The objectives, which screw into the bottom of the body tube 
or nose piece. They consist of a system of two, three or four lenses, 
some of which are simple, others compounded of a convex crown 
lens and a concave flint lens. Objectives like oculars are usually 
designated by numbers or by figures, as etc., or in milli- 
meters, which represent focal lengths. 
The smaller the number or fraction representing the focal length 
of an objective, the greater is its magnifying power. 
Objectives are either dry lenses or itnmersion lenses. If an air 
space be present between the objective and the object, the lens is 
called a dry one; if a liquid is present between the objective and the 
object, the lens is called an immersion lens. If this liquid be oil 
the objective is called an oil immersion objective; if water, a water 
immersion objective. 
Some microscopes are fitted up with a nose piece, capable of carry- 
ing two or three objectives, which may be revolved into place at the 
lower end of the body tube. Others have a condenser which is 
employed to concentrate the light upon the object examined. 
B. Its Use: 
1. Place the microscope on the table with the pillar nearest you. 
2. Screw the objectives into the nose piece and slip an ocular into 
the upper end, if not already on instrument. Turn the lowest power 
objective into position. 
3. Find the light by looking into the ocular (eye piece) and at the 
