Microscopical Essays. 



47 



fo placed, that the image may be in it's focus on one fide, and 

 the eye at the fame diftance on the other. The rays of each pen- 

 cil will be parallel, after palling out of the glafs, till they reach 

 the eye at E, where they will begin to converge by the refraaive 

 powers of the humours ; and after having crofted each other 

 in the pupil, and palled through the chrylialline and vitreous 

 humours, they will be collected in points on the retina, and form 

 a large inverted image thereon. 



It will be eafy, from what has been already explained, to un- 

 derftand the reafon of the magnifying power of a compound 

 microfc r e. The objea is magnified upon two accounts ; firft, 

 becaufeif we viewed the image with the naked eye, it would 

 appear as much larger than the objea, as the image is really 

 larger than it, or as the diftance f R is greater than the diftance 

 f b ; and fecondly, becaufe this piaure is again magnified by the 

 eye glafs, upon the principle explained in the foregoing article 

 onvifton, by fingle micro fcopes. 



But it is to be noted, that the image formed in the focus of a 

 lens, as is the cafe in the compound mierofcope, differs from the 

 real objea in a very effential particular ; that is to fay, the light 

 being emitted from the objea in every direction, renders it vifible 

 to an eye placed in any pofition ; but the points of the image 

 formed by a lens, emitting no more than a fmall conical body of 

 rays, which arrives from the glafs, can be vifible only when the 

 eye is fituate within it's confine. Thus the pencil, which 

 emanates from o in the objea, and is converged by the lens to 

 M, proceeds afterwards diverging towards H, and therefore, 

 never arrives at the lens F G, nor enters the eye at E. But the 



pencils 



