82 MICROSCOPES. « 



at the bottom of the eye, we can never reflect, without wort-* 

 der, upon the smalhiess, yet correctness of the picture, the 

 subtilty of the touch, the fineness of the lines. A land- 

 scape of five or six square leagues is brought into a space 

 of half an inch diameter ; yet the multitude of objects 

 which it contains are all preserved ; are all discriminated id 

 their magnitudes, positions, figures, colours. 



Microscopes are instruments for viewing small objects ; 

 and they apparently magnify them, because they enable us 

 to see them nearer than with the naked eye, without affect- 

 ing the distinctness of vision. The distance from the naked 

 eye, at which most persons are supposed to see small objects 

 best, is about seven inches ; but by the help of convex glasses, 

 we are enabled to view things clearly at a much shorter dis- 

 tance than this ; for the nature of a convex lens is, to render 

 an object distinctly visible to the eye at the distance of it| 

 focus. With a knowledge of this fact, we may easily de^ 

 termine the magnifying powers of glasses employed in Sin-^ 

 glc Microscopes, which are small double convex lensesj 

 having the object placed in the focus, and the eye at thq 

 same distance on the other side. If rays of light from aioi 

 object are converged to a point at the distance of one inctj 

 from the centre of the glass, or, in other words, if the focali 

 distance of the lens is one inch, an object may be seert 

 through that lens at one inch distance from the eye, and i| 

 will appear, in its diameter, — since the natural sight is seve^ 

 inches, — seven times larger than to the naked eye. But 

 the object is magnified every way equally, in length as we 

 as breadth, we must square this diameter, to know real! 

 how much the object appears enlarged ; and we shall thu» 

 find that its surface is magnified forty-nine times. If wfls 

 suppose the focus of a convex lens to be at one-tenth of ang 

 inch distant from its centre, in seven inches there arc^ 

 seventy such tenths of an inch ; and an object therefore maj^ 

 be seen through this lens seventy times nearer than it canjl 

 distinctly, by the naked eye. It will consequently appeal^ 

 seventy times longer and seventy times broader than it doe* 

 to common sight; and as seventy multiplied by seventyfj 

 makes four thousand nine hundred ; so many times it really^ 

 appears magnified. Those lenses, therefore, which have thej 

 shortest focus, will magnify the object most. Single micro-^ 

 scopes of the greatest power may be made with a very small! 



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