THE MICROSCOPE. 



" ILLUMINATION, by cups or silver specula, does not produce these 

 illusions, because they create no shade the whole object is one 

 mass of intense light ; other false perceptions are, however, occa- 

 sioned by them. Thus, all globular bodies, having polished sur- 

 faces, reflect an image of the cups, and the spout, if there is one, 

 appears as a dark spot in the centre. The eyes of insects, illumi- 

 nated in this way, show the semblance of a pupil in the centre of 

 each lens, which deception may be verified by examining 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 reflec- 

 tion of light, in all the conclusions we draw from the evidence 

 even of the very best instruments, used with every possible 

 precaution. 



" Lastly, it must be observed, that in using microscopes, we 

 must never attempt to verify an object concerning 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 employed, even with a short body ; for a microscope 

 only shows 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, unhss they are obtained 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 uncontrolled operation. Single and compound magni- 

 fiers, having 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 testi- 

 mony ; 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." 



MEASUREMENT OF OBJECTS. 



54. The determination of the real magnitude of microscopic 

 objects, and that of the magnifying power of the instrument, are 

 problems closely connected but not identical. Either may be 

 solved independently of the other. 



55. If two objects be placed at the same time within the field 

 of view, the real magnitude of one of which is known, that of the 

 .other may be at least approximately estimated by comparison. 



50 



