GENERAL BIOLOGY 



upon the object examined through a hole in the flat stage upon 

 which the object is placed. This hole can be made of various di- 

 ameters by means of the iris diaphragm. A lens for concentrating 

 light, and known as a condenser, is placed between the mirror 

 and the stage. The lens at the upper end of the tube is the ocular 

 or eye-piece ; there are two oculars, of different magnifying power. 

 The combination of lenses at the lower end of the tube is the 

 objective; in this microscope there are two objectives of different 

 magnifying power, one marked 3, the other 6; the former (low 

 power objective) is in focus, i. e., gives a clear image of the ob- 

 ject examined when its lower end is about */$ in. above the object, 

 the latter (high power objective) is in focus when it is about ^ in. 

 above the object. The objectives are carried upon a nose-piece, by 

 revolving which either one of them may be brought to lie at the 

 lower end of the tube. The tube is really double, one tube being 

 telescoped within another. By holding the body firmly in the left 

 hand and taking hold of the projecting brass ring just below the 

 eye-piece with the right, the inner tube may be drawn out some 

 distance, thus lengthening the body and increasing the magnifica- 

 tion. The length of the tube without lenses and nose-piece is 150 

 mm. and it can be drawn out to 195 mm. ; with the nose-piece the 

 tube is 10 mm. longer. A table of magnifications of the different 

 lenses with a given tube length is found on the inside of each mic- 

 roscope case. 



B. USE. i. Reflect light, from white clouds if possible, upon 

 the object. Where all the light is needed, use concave mirror; 

 where light is intense and a low magnification is required, use plane 

 mirror. 



2. Use smaller diaphragms with higher powers. 



3. To focus lenses upon an object, use the coarse, then the fine 

 adjustment; the former movement is by means of a rack and 

 pinion. The rack is the toothed plate along the back of the tube, 

 the pinion is a smal cog wheel which fits into the rack and is turned 

 by the two milled wheels on each side of the tube. The fine adjust- 

 ment is by means of a milled screw head back of the pinion. If 

 the fine adjustment screw is turned in the direction in which a 

 clock's hands move the tube is lowered, turned in the reverse direc- 

 tion it is raised. In using high power turn the tube down nearly 

 to the object, and then, while looking through the microscope, 

 bring the object into focus by slowly turning the tube upward. 



