362 



WELLS'S NATURAL PHILOSOPHY. 



simple form, consists of two lenses, so arranged that 

 the second lens magnifies the image formed by the first 

 lens, or simple microscope. In this way the image of 

 the object is examined by the eye, and not the object 

 itself. 



The first of these lenses is called the object- 

 glass, or objective, since it is always directed 

 immediately to the object, which is placed 

 very near it ; and the latter the eye-glass, or 

 eye-piece, inasmuch as the eye of the observer is applied 

 to it to view the magnified image of the object. 



Fig. 295. 



How are the 



lenses of a 

 compound mi- 

 cr iscope desig- 

 nated ? 



Fig. 295 illustrates the magnifying principle of the compound microscope. 

 O represents the object-glass placed near the object to be viewed, A B, and 

 G, the eye-glass placed near the eye of the observer, E. The object-glass, 0, 

 presents a magnified and inverted image, a b, of the object at the focus of tho 

 eye-glass, G. The image thus formed, by means of the second lens or eye- 

 glass, G, is magnified and brought to the eye at E, so as to appear under the 

 enlarged visual angle, A' E B'. If we suppose the object-glass, 0, to have a 

 magnifying power of 25 — that is, if the image a h equals 25 A B, and the 

 eye-glass, G, to have a magnifying power of 4 — then the total magnifying 

 power of the microscope will be 4 times 25, or 100; that is to say, tho 

 image will appear 100 times the size of the object. 



Fig. 296 represents the most approved form of mounting the lenses 

 which compose a compound microscope. The tube, A, which contains in 

 its upper part the eye-glass, slides into another tube, B, in the bottom of 

 ■which the object-glass is fixed ; this last tube also moves up and down in 

 the stand, C, and in this way the lenses in the tubes may be adjusted to tho 

 proper distance from each other and the object. M is a mirror for reflecting 

 light upon tho object, and S a support on which the object to be examined 

 IS placed. 



