MAGNIFYING GLASSES. 



are sometimes called simple microscopes ; but the term microscope 

 is more generally applied to that class of optical instruments which 

 consists of a combination of lenses, which are applicable to the 

 examination of the most minute objects, and with amplifying 

 powers much more extensive. 



2. Magnifiers are very variously mounted according to the uses 

 to which they are applied. The more simple forms, and those 

 which have the least amplifying power, consist of a single lens, 

 which is either convex on both sides, or plano-convex, or which 

 may be concave on one side and convex on the other, provided the 

 convexity be greater than the concavity. In fine, whatever be its 

 form, it is essential that convexity shall prevail. 



3. These glasses are of very extensive use in the arts. In 

 all cases in which the objects operated upon are minute, the inter- 

 position of a magnifier is found advantageous, and often indis- 

 pensable ; thus, they are invariably used in different mountings 

 by watch-makers, jewellers, miniature-painters, engravers, and 

 others. 



4. To render our explanation of these very convenient instru- 

 ments intelligible, it will be necessary that the reader should be 

 previously more or less familiar with what has been already ex- 

 plained in our Tracts on the Eye, on Optical Images, and on Spec- 

 tacles ; we shall, therefore, take for granted, that the contents of 

 these Tracts are known to the reader. 



"We know no subject respecting which more inexact and erro- 

 neous notions prevail, than the amplification or magnifying effect 

 produced by all optical combinations, from the simple convex lens 

 to the most powerful microscope. The chief cause of all this con- 

 fusion and obscurity may be traced to a neglect of the proper dis- 

 tinction between visual and real magnitude. The eye, as has been 

 already explained, takes no direct cognizance of real magnitude, 

 which it can only estimate by inference and comparison with the 

 impressions of the sense of touch ; these inferences and compari- 

 sons being often attended with complicated calculations and 

 reasoning. If a proof of this be required, it may be found in the 

 universally observable fact that objects which have the same 

 visual magnitude often have real magnitudes enormously diffe- 

 rent ; thus, for example, the apparent or visual magnitudes of the 

 sun and moon are, as every one knows, equal : yet the real 

 diameter of the sun is more than 400 times that of the moon. 



5. It must be remembered that visual magnitude is determined 

 by the divergence of lines drawn from the eye to the extreme 

 limits of the object; it is measured, therefore, not like real mag- 

 nitude by miles, feet, and inches, but by degrees, minutes, and 

 seconds ; thus, while the real diameter of the moon measures 



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