

22 THE MICROSCOPE AND ITS REVELATIONS. 



of one of these Achromatic combinations for a single lens of equivalent 

 focus, even where the use of the former reveals no detail that is not dis- 

 cernible by the latter. 



3. Compound Microscope. 



26. The Compound Microscope, in its most simple form, consists of 

 only two lenses, the object-glass and the eye-glass. The former, c D (Fig. 

 13), receives the light-rays direct from the object, A B, brought into near 

 proximity to it, and forms an enlarged but inverted and reversed image, 

 A'B', at a greater distance on the other side ( 8); whilst the latter, L M, 

 receives the rays which are diverging from this image, as if they pro- 

 ceeded from an object actually occupying its position and enlarged to its 

 dimensions, and brings these 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 ( 22). It is obvious that, in the use of the very 

 same lenses, a considerable variety of magnifying power may be obtained, 

 by merely altering their position in regard to each other and to the ob- 

 ject: 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 consequently 

 be augmented; whilst, on the other hand, if the eye-glass be brought 

 nearer to the object-glass, and the object removed farther from it, the 

 distance of the image will be a much smaller multiple of the distance of 

 the object, and its dimensions proportionately diminished. We shall 

 hereafter see that this mode of varying the magnifying power of Com- 

 pound Microscopes may be turned to good account in more than one 

 mode ( 83, 84); but there are limits to the use which can be advantage- 

 ously made of it. The amplification may also be varied by altering the 

 magnifying 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 image is amplified to a much greater extent. 

 In practice, it is generally found much better to vary the power by em- 

 ploying 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 the object, and its dimensions are proportion- 

 ably larger. In whatever mode increased amplification may be obtained, 

 two things must always result from the change: the proportion of the 

 surface of the object of which an image can be formed must be dimin- 

 ished; and the quantity of light spread over that image must be propor- 

 tionally lessened. 



27. In addition to the two lenses of which the Compound Microscope 

 essentially consists, it is found advantageous to introduce another (F F, 

 Fig. 14), between the object-glass and the image formed by it; the pur- 

 pose of this lens being to change the course of the rays in such a manner, 

 that the image may be formed of dimensions not too great for the whole 

 of it to come within the range of the Eye-glass. As it thus allows more 

 of the object to be seen at once, it has been called the field-glass; -but it 

 is now usually considered as belonging to the ocular end of the instru- 

 ment the eye-glass and the field-glass being termed the Eyepiece. Va- 



