254 Me Microfcope. ' t [Book III. 



of the objeft-glafs to that of the eye-glafs *. In all 

 cafes, in whatever manner we combine our glades to 



* Microfcopes are conftruaed in t\vo different modes. The 

 one is, by the interpofition of a convex lens between the objeft 

 and the eye, to fender it diftinft at a lefs diftance than fix inches, 

 by which means its apparent magnitude increafes as the diftance 

 is diminilhed ; and the other is, by placing the object fo with re- 

 fpecl to a convex lens, that its focal image may be much greater 

 than itfelf, and contemplating that image inftead of the object. 

 The firft are called fimple or fingle microfcopes, and the latter 

 compound or double. The former is conftrufted in this manner : 

 fuppofe a fmall object fituated very near the eye, fo that the angle 

 of its apparent magnitude may be large; .then its image on the 

 retina will alfo be large ; but becaufe the pencils of rays are too 

 divergent to be collected into their foci on the retina, it will be 

 very confufed and indiftinct. Then let a convex lens be inter- 

 pofcd, fo that the diftance between it and the. object may be equal 

 to the focal length at which parallel rays would unite, and the rays 

 which diverge from the object, and pals through the lens, will af- 

 terwards proceed, and confequently enter the eye, .parallel ; they 

 will therefore unite, and form a diftindt image on the retina, and 

 the object will be clearly feen, though if removed to the diftance 

 of Cx inches, its fmallnefs would render it invifible. The moft 

 convex lenfes, having the fhorteft fecal diflance of parallel rays, 

 muft magnify the moft ; for they permit the object to approach 

 nearer the eye than thofe do which are flatter. 



A drop of water is a kind of microfcope, from its convex fur- 

 face ; for, if a fmall hole is made in a plate of metal, or other thin 

 fublbmce, and carefully filled with a drop of water, fmall objects 

 may be feen through it very diftindl, and much magnified. 



In the compound microfcope, the image is contemplated inftead 

 of the object ; it is of two kinds, the folar and the common dou- 

 ble microfcope ; in the latter, the image is viewed through a fingle 

 lens In the fame manner as the okjeJl in a fingle microfcope. The 

 folar microfcope is conftructed by placing a convex lens oppofite 

 a hole in a darkened chamber, and placing the object at a proper 

 diftarce from the lens, the pencil of light will converge to a focus 

 on a fcreen, and the pencil which proceeds from the other point 

 will converge to another locus, and the intermediate points of the 

 objedl will be formed into a picture, which will be as much larger 

 than the objedl in proportion as the diftance of the fcreen exceeds 

 that of the image from the lens. 



difcover 



