OPTICAL PRINCIPLES OF THE MICROSCOPE. 



Fig. i 1. 



Fig. 12. 



position in regard to each other and to the object; for if the eye- 

 glass be carried further from the object-glass, whilst the object is 

 approximated nearer to the latter, the image a' b' will be formed 

 at a greater distance from it, and its dimensions will consequently 

 be augmented. If on th,e other hand, the eye-glass be brought 

 nearer to the object-glass, whilst the object is removed further 

 from it, the distance of the image will be shortened, and its 

 dimensions proportionably diminished. We shall hereafter see 

 that this mode of varying the magnifying power of compound 

 microscopes may be turned to good account in more than one 



mode (§§ 43, 44) ; but there are 

 limits to the use which can be 

 advantageously made of it. — 

 The amplification may also be 

 varied by altering the magni- 

 fying power of the eye-glass ; 

 but here, too, there are limits 

 to the increase; since defects 

 of the object-glass, which are 

 not perceptible when its image 

 is but moderately enlarged, are 

 brought into injurious promi- 

 nence when the imperfect im- 

 age is amplified to a much 

 greater extent. In practice, it 

 is generally found much better 

 to vary the power, by employ- 

 ing object-glasses of different 

 foci ; an object-glass of long 

 focus forming an image, which 

 is not at many times the dis- 

 tance of the object from the 

 other side of the lens, and 

 which, therefore, is not of 

 many times its dimension ; 

 whilst an object-glass of short 

 focus requires that the object 

 should be so nearly approxi- 

 mated to it, that the distance 

 of the image is a much higher 

 multiple of that of the object, 

 and its dimensions are propor- 

 tionably larger (§ 8). In what- 

 ever mode additional amplifica- 

 tion be obtained, two things must always result from the change: 

 the portion of the surface of the object, of which an image can be 

 formed, must be diminished ; and the quantity of light spread 

 over that image must be proportionably lessened. 



21. In addition to the two lenses of which the Compound 

 Microscope essentially consists, another (Fig. 12, f f) is usually 



Diagram of simplest 

 form of C'ovipound Micro- 

 scope. 



Diagram of the com- 

 plete Compinutd Micro- 

 scope. 



