Microscopical Essays. 



53 



The fourth obftacle which arifes from „the different refrangi- 

 bility of the rays of light, and which frequently caufes fuch 

 deviations from truth in the appearance of things, that many have 

 imagined themfelves to have made furprizing difcoveries, and have 

 communicated them as fuch to the world ; when, in fact, they 

 have been only optical deceptions, owing to the unequal refrac- 

 tion of the rays. In telefcopes, this error has been happily 

 corrected by Mr. Dollond's valuable difcovery of achromatic 

 glaffes ; but how far this invention is applicable to the improve- 

 ment of microfcopes, has not yet been afcertained ; and indeed 

 there is fome reafon for fuppofmg, they cannot be fuccefsfully 

 applied to them ; fo that this obftacle remains yet to be remedied, 

 before microfcopes can be faid to have received their ultimate 

 degree of perfection. 



Of the Magnifying Powers of the Microscope. . 



We have already treated of the apparent magnitude of objects, 

 and (hewn that they are meafured by the angles under which they 

 are feen, and that this angle is greater or fmaller, according as 

 the object is nearer to, or further from, the eye; and con- 

 fequently, the lefs the diftance at which it can be viewed, the 

 larger it will appear : but from the limits of natural vifion, the 

 naked eye cannot diftinguifh an object that is very near to it ; yet, 

 when affifted by a convex lens, diftinct virion is obtained, how- 

 ever fhort the focus of the lens, and confequently, how near 

 foever the object is to the eye ; and the fhorter the focus of 

 the lens is, the greater will be the magnifying power thereof. 

 From thefe confi derations, it will not be difficult to eftimate the 

 magnifying power of any lens ufe-d as a fingle microfcope ; for 



