18 THE MICROSCOPE AND ITS REVELATIONS. 



precisely in the case of such objectives that the f cover-correction ' needs* 

 to be most exact. And although the practised microscopist has no diffi- 

 culty in making this, when the object at which he is looking (such as a 

 Diatom, a Podura-scale, or a band of Robert's ruled lines) is known to-, 

 him, yet the case is entirely different when the objecfc is altogether" 

 unknown. For in examining such an object, he may be able only to sat- 

 isfy himself after repeated trials, involving much expenditure of time 

 and patience, as to the cover-correction which gives the truest represen- 

 tation of the object; whilst, in using a ' homogeneous ' or 'oil-immer- 

 sion' objective, he is able to feel an absolute certainty that, without any 

 adjustment at all, the view which he gains of an unknown object is in 

 every respect at least equal to that which he can obtain from the best 

 'dry' or ' water-immersion ' objective, most exactly adjusted for thick- 

 ness of cover. This system has been taken up also by Messrs. Powell and 

 Lealand, who have constructed admirable ( oil-immersion ' objectives 

 ranging to 1-25 th inch focus, which, by a change of the front lens, may 

 also be used ' dry.' 



21. We are now prepared to enter upon the application of the Optical 

 principles which have been explained and illustrated in the foregoing 

 pages, to the construction of Microscopes. These are distinguished as 

 Simple and Compound; each kind having its peculiar advantages to the 

 Student of Nature. Their essential difference consists in this: that in 

 the former, the rays of light which enter the eye of the observer proceed 

 directly from the object itself, after having been subjected only to a 

 change in their course; whilst in the latter, an enlarged image of the 

 object is formed by one lens, which image is magnified to the observer by 

 another, as if he were viewing the object itself. The Simple Microscope 

 may consist of a single lens; but (as will be presently shown) it may be 

 formed of two, or even three: these, however, being so disposed as to pro- 

 duce an action upon the rays of light corresponding to that of a single 

 lens. In the Compound Microscope, on the other hand, not less than 

 two lenses must be employed: one, to form the enlarged image of the ob- 

 ject, immediately over which it is placed, and hence called the object- 

 glass; whilst the other again magnifies that image, and, being interposed 

 between it and the eye of the observer, is called the eye-glass. A perfect 

 Object-glass, as we have seen, must consist of a combination of lenses; 

 and the eye-glass is best combined with another lens interposed between 

 itself and* the object-glass, the two together forming what is termed an 

 eye-piece ( 27). These two kinds of instrument need to be separately 

 considered in detail. 



2. Simple Microscope. 



22. In order to gain a clear notion of the mode in which a Single- 

 Lens serves to 'magnify' minute objects, it is necessary to revert to the 

 phenomena of ordinary Vision. An Eye free from any defect has a con- 

 siderable power of adjusting itself in such a manner as to gain a dis- 

 tinct view of objects placed at extremely varying distances; but the 

 image formed upon the retina will of course vary in size with the dis- 

 tance of the object; and the amount of detail perceptible in it will 

 follow the same proportion. To ordinary eyes, however, there is a 

 limit within which no distinct image can be formed, on account of the 

 too great divergence of the rays of the different pencils which then enter 

 the eye; since the eye is usually adapted to receive, and to bring to a focus, 

 rays which are parallel or but slightly divergent. This limit is vari- 



