Small Aperture Large Aperture 

 Lens. Lens. 



Fig. 5. — a = angular aperture. 



A SIMPLE DESCRIPTION OF THE MICROSCOPE 15 



enter the instrument (see Fig. 5). It was soon found tliat 

 when this was the case, although great magnifying power could 

 be obtained, fine detail could not be seen, but merely a repre- 

 sentation on a larger scale of the coarse structure which could 

 readily be seen with a lower magnifying power. 



In order that an advantage should be obtained from the use 

 of higher magnifying power, it was necessary to admit into the 

 microscope a correspondingly larger cone of light from each point 

 of the object, as unless this 

 were done, no advantage could 

 be obtained in the observation 

 of fine details. Such a plan 

 had the further advantage that 

 it collected a larger amount of 

 light and rendered the object 

 more brilliant. The size of the 

 cone of light admitted into the 

 microscope from each point of 

 the object is called the aperture 

 {a, Fig. 5). It is expressed 

 either by the angle of the cone 

 of light entering the micro- 

 scope or by a figure called the numerical aperture, or N.A. 



The aperture is of such paramount importance, that the limit 

 of what can be seen with the microscope does not depend upon 

 what magnifying power can be obtained, but upon what size 

 cone of light can be collected from the object by means of the 

 object glass ; and lenses can be made with a much higher magnify- 

 ing power, but they cannot be made with a larger aperture, than 

 those in use at the present time. 



The aperture, therefore, has a direct bearing upon the best 

 method of increasing magnifying power, because, if an object 

 glass can only admit a certain aperture of light, the use of an 

 eyepiece does not alter this property, and therefore to increase 

 the magnifying power by high eyepieces is of no service, when 

 carried beyond that power which is sufficient to enable the detail 

 that can be shown by the aperture of a particular object glass 

 to be seen. 



The best method of increasing the magnifying power is, 

 therefore, by changing the object glass. Most object glasses 

 have sufficient aperture to allow of the use of an eyepiece of as 

 high a power as 15, but, in general, magnification of more than 

 10 by means of the eyepiece should only be used in special cases, 

 and the object glass should be changed rather than the eyepiece. 



The same reason makes it undesirable to depend for increased 

 magnifying power upon extending the drawtube of the micro- 

 scope, and the more so in this case because the object glass can 

 only be constructed to work at its best with a particular length 



Limit of 

 vision 

 dependent 

 on aperture. 



Best method 

 of increasing 

 magnifying 

 power. 



standard 

 length of 

 body. 



