OBSERVING AND MANAGING ENGISCOPE. 183 



wise the perspiration from the eye will be perpetually 

 condensing on the eye-glasses, &c. and greatly impede 

 vision. 



Always begin to examine your object with the lowest 

 power you have, unless it is very minute ; it may be laid 

 down as a general rule, that large objects require low 

 powers, and small ones high powers ; that the low 

 powers show the whole or general view of an object, and 

 the high ones only its parts in succession. 



Never use high powers unless absolutely necessary ; 

 for as the power increases, so does the difficulty of 

 finding the object, and of adjusting the focus. The 

 colours also grow fainter and more diluted, and the 

 shades darker and darker, until all ease and satisfaction 

 in observation, together with all certain vision, fade 

 away, and so very small a portion of the object is seen, 

 that it is difficult to know what we are looking at. Re- 

 collect that a really good aplanatic object-glass shews every 

 thing ivith very low powers ; in fact in this property its 

 goodness and beauty consist : do not plume yourself, 

 therefore, upon having an instrument which shews 

 objects with very high powers, but with very low ones. 



Where, however, from the minuteness of an object, it 

 becomes really necessary to use a high power, always 

 select a small deep object-glass, and use it along witb a 

 shallow or long eye-piece, in preference to using an object 

 glass of low power, with a short or powerful eye-piece, 

 for magnifying power is much more valuable and effec- 

 tive when derived from the object-glas^ than from the 



