COMPOUND MICROSCOPE. 23 



3. Compound Microscope. 



20. In its most simple form, this instrument consists of only 

 two lenses, the Object-glass and the Eye-glass : the former, c d 

 (Fig. 13), receiving the rays of light direct from the object, A b, 

 which is brought into near proximity to it, forms an enlarged and 

 inverted image a' b' at a greater distance on the other side ; whilst 

 the latter, l m, receives the rays which are diverging from this 

 image, as if they proceeded from an object actually occupying its 

 position and enlarged to its dimensions, and these it brings to the 

 eye at e, so altering their course as to make that image appear far 

 larger to the eye, precisely as in the case of the Simple microscope 

 (§ 17). — It is obvious that, by the use of the very same Lenses, a con- 

 siderable variety of magnifying power may be obtained, by merely 

 altering their position in regard to each other and to the object ; for 

 if the'Eye-glass be carried farther 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 con- 

 sequently be augmented. If, on the other hand, the Eye-glass be 

 brought nearer to the Object-glass, whilst the object is removed 

 farther 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 Micro- 

 scopes may be turned to good account in more than one mode 

 (§§ 63, 64); but there are limits to the use which can be advan- 

 tageously made of it. The amplification may also be varied by 

 altering the magnifying power of the Eye-glass ; but here, too, 

 thei-e are limits to the increase ; since defects of the object-glass 

 which are not perceptible when its image is but moderately en- 

 larged, are brought into injurious prominence when the imperfect 

 image is amplified to a much greater extent. In practice, it is 

 generally found much better to vary the power by employing 

 Object-glasses of different foci ; an object-glass of long focus form- 

 ing an image which is not at many times the distance 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 approximated to it, 

 that the distance of the image is a much higher multiple of that of 

 the object, and its dimensions are proportionably larger (§ 8). — In 

 whatever mode increased amplification may 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 propor- 

 tionably lessened. 



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

 Microscope essentially consists, another (Fig. 14, pp) is usually 

 introduced between the Object-glass and the image formed by it. 

 The purpose of this lens is to change the course of the rays in such 



