1 8 APPLIED PHYSIOLOGY 



of an object. Thus a microscope making a cell appear 

 100 times as broad as it really is, is said to magnify 100 

 diameters. But the length and thickness are also magni- 

 fied. So the surface of the cell is made 100 x 100 or 

 10,000 times as large, while its bulk is 100 x 100 x 100 

 or 1,000,000 times as large. A table accompanying each 

 instrument tells the power of each combination of lenses. 



15. Arrangement of the light. A small mirror placed at the 

 lower part of the microscope throws light through the object, for other- 

 wise there would not be sufficient light to spread over its magnified 

 surface. The mirror can be tilted so as to catch the light from any 

 direction. Objects usually show best when they are lighted only suf- 

 ficiently to show their outlines. A stronger light may pass through 

 extremely small objects so that they do not show at all. Each micro- 

 scope usually has a device for varying the size of the aperture in the 

 plate upon which the specimen rests, thus again regulating the amount 

 of light. It is usually not best to use an amount of light which makes 

 the field of view brilliant. 



16. Focusing. The tube carrying the lenses can be moved up 

 and down by means of a small wheel. Arranging the distance of the 

 lens from the specimen is called focusing. An objective of high mag- 

 nifying power must be much nearer to the specimen than one of low 

 power. Thus an objective magnifying 500 diameters must be about 

 Y 1 ^ of an inch from the specimen, while one magnifying 50 diameters 

 would be over half an inch distant. For high magnifying powers 

 the focusing must be very exact. So a second wheel is provided 

 which moves the tube very slowly. This wheel is called the fine ad- 

 justment in distinction from the other wheel or coarse adjustment. 

 The finger of the observer should always be upon the fine adjustment, 

 turning it back and forth so as to observe now the top and now the 

 deeper portions of the specimen, for it is magnified in depth as well as 

 in breadth. 



It is often very difficult to find a very small specimen with a high 

 power lens, for the space in which it lies is magnified to several feet in 

 diameter. A good plan is to use a low power lens for finding the speci- 

 men, and then after bringing it to the center of the field, to substitute 

 the high power lens. 



