g6 Microscopical Essays. 



at D. Suppofe the veffel to be now filled with water, and the 

 fhadow will withdraw to d ; the ray of light, inftead of proceed- 

 ing to D, being refracted or bept to d. And there is no doubt 

 but that an eye, placed at d, would fee the candle at e, in the 

 direction of the refracted ray d A. This is alfo confirmed by the 

 following pleafing experiment. 



Lay a {hilling, or any piece of money, at the bottom of a 

 bafon ; then withdraw from the bafon, till you lofe fight of the 

 fhilling ; fill the bafon nearly with water, and the fhilling will be 

 feen very plainly, though you are at the fame diftanee from it. 



Experiment 3 . Place a flick over a bafon which is filled with 

 water ; then reflect the fun's rays, fo that they may fall perpen- 

 dicularly on the furface of the water ; the fhadow of the flick will 

 fall on the fame place, whether the veffel be empty or full. 



What has been faid of water, may be applied to any tranfc 

 parent medium, only the power of refraction is greater in fome 

 than in others. It is from this wonderful property, that we 

 derive all the curious effects of glafs, which make it the fubject of 

 optics. It is to this we owe the powers of the microfcope and 

 the telefcope. 



To produce thefe effects, pieces of glafs are formed into given 

 figures, which, when fo formed, are called lenfes. The fix un- 

 der-mentioned figures, are thofe which are moft in ufe for optical 

 purpofes* 



