62 On the UNEQUAL 



It appears from the preceding experiments, that in com- 

 pound object-glaffes of crown and flint-glafs, there is only a 

 partial correction of the aberration from unequal refrangibi- 

 lity, and therefore in them, and others of that kind, the aper- 

 tures and magnifying powers muft only be increafed in a fub- 

 duplicate ratio of the increafe of length, as in fingle lenfes. 



I will not pretend to (late with abfolute certainty the pre- 

 cife aperture which an achromatic telefcope of a given length 

 ought to have. This muft be determined by experience. If 

 two inches betaken for the greateft aperture" which ought to be 

 given to a telefcope of this kind two and thirty inches long, 

 then three inches will be too much for one of forty-two inches, 

 as hath been already obferved. But whichfoever of thefe 

 lengths and apertures be taken as the ftandard, it is certain, 

 that if we would avoid a greater degree of that indiftinctnefs 

 which is occafioned by the aberration from difference of refran- 

 gibility, the aperture and magnifying power muft not be in- 

 creafed in a greater proportion than the fquare root of the in- 

 creafed length. Befides, therefore, that this imperfect: correc- 

 tion renders fuch telefcopes incapable of bearing high magni- 

 fying powers for thofe of moderate lengths, large inftruments, 

 if they were to be attempted, would ftill be unmanageable, on 

 account of their immoderate lengths. The focal length of an 

 object-glafs of this kind, four feet in diameter, would require 

 to be upwards of fifteen hundred feet, in order to enable it to 

 bear the magnifying power adapted to that aperture, with the 

 fame diftinctnefs that is found in an object-glafs two inches in 

 diameter, and thirty-two inches in focal length. But when 

 the aberration from difference of refrangibility is totally re- 

 moved, the focal length of an object-glafs four feet in diame- 

 ter, need not exceed twenty feet. 



Having mentioned to fome friends the imperfect correction 

 of the aberration from difference of refrangibility, which is 

 obtained bv the common combination of two mediums which 



id 



differ 



