Microscopical Essays. 



out them. The advantages procured by the magnifying power, 

 would be exceedingly circumfcribed, if they were not accom- 

 panied by the latter : for if the fame quantity of light is diflufed 

 over a much larger furface, it's force is proportionably diminifhed ; 

 and therefore the object, though magnified, will be dark and 

 obfcure. Thus, fuppofe the diameter of the object to be enlarged 

 ten times, and confequently the furface one hundred times, yet, 

 if the focal diftance of the glafs was eight inches, (provided this 

 was poffible) and it's diameter only about the fize of the pupil of 

 the eye, the object would appear one hundred times more obfcure 

 when viewed through the glafs, than when it was feen by the 

 naked eye ; and this even on the fuppofition, that the glafs tranf- 

 mitted all the light which fell upon it, which no glafs can do. 

 But if the glafs was only four inches focal diftance, and it's 

 diameter remained as before, the inconvenience would be vaftly 

 diminifhed ; becaufe the glafs could be placed twice as near the 

 obje£t as before, and would confequently receive four times as 

 many rays as in the former cafe, and we mould therefore fee it 

 much brighter than before. By going on thus, diminifhing the 

 focal diftance of the glafs, and keeping it's diameter as large as 

 poffible, we {hall perceive the objed proportionably magnified, 

 and yet remain bright and diftinft. Though this is the cafe in 

 theory, yet there is a limit in optical inftruments, which is foon 

 arrived at, but which cannot be palled. This arifes from the 

 following circumftances. * 



1. The quantity of light loft in paffing through the glafs. 



2. The 



* Encyclopedia Br tannica, vol. viii. p. 5635. 



