Microscopical Essays, 



2. The diminution in the diameter of the glafs or lens itfelf, by 

 which it receives only a final! quantity of rays. 



3. The extreme fhortnefs of the focal diflance of great mag- 

 nifiers, whereby the free accefs of the light to the object we wifh 

 to view is impeded, and confequently the reflection of the light 

 from it is weakened.. 



4. The aberration of the rays, occafioned by their different 

 refrangibility. 



To make this more clear, let us fuppofe a lens made of fuch 

 dull kind of glafs, that it tranfmits only one half the light that 

 falls upon it. It is evident, that mppofing this lens to be of four 

 inches focus, and to magnify the diameter of the object twice, 

 and it's own breadth equal to that of the pupil of the eye, the 

 object, will be four times magnified in furface, but only half as 

 bright as if it was feen by the naked eye at the ufual diflance ; for 

 the light which falls upon the eye from the object at eight inches 

 diflance, and likewife the furface of the object in it's natural fize, 

 being both reprefented by 1, the furface of the magnified object 

 will be 4, and the light which makes it vifible only 2 ; becaufe 

 though the glafs receives four times as much light as the naked 

 eye does at the ufual diflance of diflincl: virion, yet one half is loft 

 in palling through the glafs. The inconvenience, in this refpecl, 

 can only be removed fo far as it is pofhble to increafe the tranf- 

 parency of the glafs, that it may tranfmit nearly all the rays 

 which fall upon it ; and how far this can be done, has not been 

 yet afcertained. 



