The Use of a Simple JSIicroscope. 



19 



The disadvantage of small glasses of short focus is that 

 they have to be held close to the eye, and the object close 

 to the glass, Avhich causes a degree of strain to the eyes that 

 is both disagreeable and dangerous to the eyesight. These, 

 however, are now almost" wholly superseded by the com- 

 pound microscope, wherein this defect is entirely overcome. 

 A much greater additional advantage is also gained by substi- 

 tuting the latter for the sim/le magnifier. When the utility of 

 the magnifying-glass has been once found out, it almost in- 

 variably leads to a craving for greater power, so as to obtain a 

 still further enlargement and better view of the object under 

 examination. The principle of the single glass does not admit 

 of any very great increase of this magnifying power, while 

 with the compound arrangement the limit of the enlargement is, 

 as yet, scarcely known, and as now constructed, modern achro- 

 matic instruments* are so contrived as to provide the highest as 

 well as extremely low powers. 



Since the common hand lens can be of but very limited use to 

 ihe farmer, and is wholly inadequate to most of the purposes for 

 which he ought to apply it, and as the manufacture of more per- 

 fect instruments has been brought to a great degree of perfection, 

 at such comparatively small cost as to place them within the 

 means of nearly every student, it has been 

 deemed desirable to advert to their con- 

 struction. 



With a simple magnifying lens the ob- 

 ject itself is seen through the glass. Not 

 so, however, with the compound micro- 

 scope. A convex lens has the property of 

 forming a picture of any object presented 

 to it, at some specified distance on its op- 

 posite side ; thus in the annexed diagram 

 the picture B is so much larger than A, 

 the object itself, as the distance is greater 

 between B and C (the lens) than between 

 A and C. In the compound microscope, 

 it is not the object itself, but this greatly 

 enlarged picture, B, that is seen, being 

 again magnified by another single lens ; 

 thus its twofold action of enlargement 

 gives it a claim to be termed " com- Fig. i6. 



pound." 



* Instruments having their imperfections corrected, although more strictly 

 ■' without colour." 



c 2 



