4 TUB USE OF THE MICROSCOPE 



perforated in the middle by a hole, the size of which can be 

 varied by means of diaphragms. Through this hole light is 

 directed on the object to be examined by means of a mirror 

 attached to the stand below the stage. Above the stage the 

 stand supports a vertical tube, in which the body of the micro- 

 scope slides up and down. 



The body is a tube, in the upper end of which is placed a 

 combination of lenses, known as the eyepiece, while to the lower 

 end is screwed another combination of lenses the objective. 



A microscope is usually provided with a couple of eyepieces 

 and a couple of objectives of different magnifying power. An 

 objective magnifying only a small number of times is called a 

 low power; one magnifying many times (200 diameters or more), 

 a high power. Similarly eyepieces are spoken of as high or low 

 according to their magnifying power. 



In order that an object may be seen clearly the objective 

 must be at a certain definite distance from the object, this 

 distance (the " focal length ") varying with different objectives, 

 and to a slight extent with different observers. The higher 

 the power employed the closer must the objective be brought 

 to the object. As the position of the object on the stage of the 

 microscope is fixed, this distance is regulated by moving the 

 body of the microscope up and down in the tube in which it slides. 



This process of focussing is effected in two ways: 



(1) By simply sliding the body up and down by hand in the 

 stand, or by screwing it up and down with a rack and pinion, 

 according to the type of microscope employed. This is known 

 as the coarse adjustment. The sliding of the body should be 

 performed with a slightly screwing motion, and 3an only be used 

 when low powers are being employed. 



(2) With high powers the objective has to be brought so 

 close to the object that a more delicate method of adjustment 

 is necessary. This^ne adjustment is effected by a screw with a 

 milled head placed at the top of the vertical pillar forming the 

 stand. By turning the head from right to left, in the direction 

 of the hands of a watch, the body of the microscope is lowered 

 and the objective brought nearer to the object : by turning in 

 the reverse direction the objective is raised. 



In using the microscope attend to the following rules : 

 1 . Always examine an object first with the low power. Having 

 adjusted the eyepiece and objective, direct the light up the tube 



