MICROSCOPIC ILLUSTRATIONS. 263 



small globules of mercury in the same manner. Spherical 

 bodies, with bright surfaces, will even, on some occasions, 

 reflect an image of the object-glass and its setting, on the 

 same principle ; so that we must perpetually consider the 

 laws of the refraction and reflection of light, in all the 

 conclusions we draw from the evidence even of the very 

 best instruments, used with every possible precaution. 



20. Lastly, it must be observed, that in using engis- 

 copes, we must never attempt to verify an object concern- 

 ing which we are uncertain, by increasing the depth of the 

 eye-glass immoderately, so as in this way to obtain a very 

 high power. A negative eye-glass, of about one-fourth of 

 an inch focus, is the deepest which should ever be em- 

 ployed, even with a short body ; for an engiscope only 

 shews a. picture of an object, and the more it is amplified 

 the more its imperfections are developed. It is, on this 

 account, much safer to trust to moderate powers in these 

 instruments, in preference to high ones, unless they are ob- 

 tained through the medium of the depth and power of their 

 objective part. It is the nature of deep eye-pieces to cause 

 all luminous points to swell out into discs, and to render 

 the image soft, diluted, and nebulous ; at length all certain 

 vision fades away, and the imagination is left to its uncon- 

 trolled operation. Single and compound magnifiers, hav- 

 ing to deal with the real object, may be made of any power 

 which can be used ; and if our eyes are strong, and habi- 

 tuated to their use, we may place great reliance on their 

 testimony ; but we must never allow them to persuade us 

 to believe marvels which are manifestly impossible, or 

 contrary to the known laws of nature and right reason. 



