CHAPTER IV. 



HOW TO USE THE MICROSCOPE. 



MANY persons imagine that the value of a microscope is in 

 proportion to the apparent size of an object seen through it. 

 This, however, is a mistake. The greater the magnifying 

 power of an instrument, all other things being equal, the 

 greater is the difficulty of finding a minute object on the stage, 

 and of adjusting the focus. The light, too, transmitted from 

 the mirror, becomes less intense, and the view less satisfactory 

 with the use of high powers. For the majority of objects, a 

 low or medium power is always preferable, on account of the 

 greater extent of the field of view. The test objects, however, 

 and the minute structure of any delicate tissue, &c., require 

 very considerable amplification in order to exhibit them satis- 

 factorily. When this is the case, the increase of power should 

 be given by the employment of an object-glass of shorter focal 

 length, in preference to the use of a more powerful eye-piece. 



Sir David Brewster gives the following rules for microscopic 

 observations. 



1. The eye should be protected from all extraneous light, 

 and should not receive any of the light which proceeds from 

 the illuminating centre, excepting what is transmitted through 

 or reflected from the object. This rule will illustrate the use 

 of the diaphragm under the stage of the microscope. 



2. Delicate observations should not be made when the fluid 

 which lubricates the cornea of the eye is in a viscid state. 



