20 



The Use of a Simple Microscope. 



The marginal figure shews how this effect is accomplished 



in the arrangement known as the " body" of the instrument. 



^ The object, A, being properly placed, and 



I : , sufficient light thrown upon it, an enlarged 



I ,<^.3sHJ picture of it is seen at B. The large lens 



'^ '^ just below this picture is termed the field lens, 



because it regulates the rays of light forming 

 the picture, while the topmost or eye-glass is 

 merely a single magnifying lens, through 

 Avhich the picture B is seen and further 

 enlarged. The distance of B from the eye- 

 glass is regulated by moving the object (A) 

 nearer to, or farther from the lens, which is 

 termed the "object glass" just above it. The 

 lenses which are thus employed to produce 

 the picture of the object, are mounted in 

 combinations for each power, each set being 

 contained in a separate brass fitting, which 

 screws on at the lower end of the tube, and 

 great care must be taken not to displace or 

 get these soiled by the fingers. The quality 

 of a microscope depends greatly upon the qua- 

 lity of its object-glasses, and these are very 

 often purchased separately — a series of two or 

 three being generally required — a 1-inch and 

 ^-inch where 2 only are used ; a 2-inch or 1^, 

 a |.rd or i-inch, and a i or ^ where 3 ; and ith 

 added where a higher power still is desired. 

 The simple magnifier which is then used to 

 enlarge the picture in the eye is termed the eye- 

 piece, the form represented in the upper por- 

 tion of Fig. 17, and again in Fig. 18, two 

 or three sizes of which are generally supplied 



^| |j=jr= -' with the best microscopes. The upper glass of 



L ; . L this must be carefully wiped whenever the in- 



strument is used, as dust, and damp from the 

 eye tend to render it misty. 



All additional arrangements of a microscope 

 are with a view to facilitate the proper placing 

 and illuminating of the object to be seen, to- 

 gether with the measurement of its length, 

 breadth, and thickness. 



Different magnifying powers are obtained by 

 changing the lower lens or object glass, a series of which is 

 represented in Fig, 18, and also by substituting shorter, that is, 



Fig. IT. 



