RKFRANGIBILITT of LIGHT. 57 



From infpecting the tables of the lengths and apertures of 

 telefcopes with fimple object-glafTes, it will appear, that the re- 

 quired length for an aperture of two inches is about thirteen 

 feet. This exceeds two feet and an half, the length given to an 

 achromatic telefcope, whofe object- glafs is two inches in dia- 

 meter, between five and fix times. The length of the flandard 

 Hugenian telefcope, whofe aperture is three inches, is thirty 

 feet. This is between eight and nine times the length of an 

 achromatic telefcope, the aperture of which is likewife three 

 inches, and its length three and a half feet. But if the aber- 

 ration from unequal refrangibility be diminifhed to the fame 

 degree as in the thirty inch telefcope, the length muft be in- 

 creafed, from three and a half feet to about five and a half. 

 For its length muft be to thirty inches, the length of the two 

 inch aperture, as the fquare of two to the fquare of three, and 

 then the telefcope with the fimple object-glafs will only exceed 

 it in length between five and fix times as before. 



The obfervations which have been mentioned put it beyond 

 a doubt, that the limit to the apertures and magnifying powers 

 of what have been improperly called achromatic telefcopes, is 

 the very fame which limits the performance of telefcopes with 

 fimple object-glafTes, namely, the unequal refrangibility of 

 light ; and it would feem, that the aberration from this caufe 

 may be diminifhed, by a combination of lenfes of crown and 

 flint glafs, between five and fix times. 



Sir Isaac Newton, by accurate experiments, hath deter- 

 mined the diameter of the leaft circular fpace within which 

 parallel rays of all kinds can be collected by a fimple lens, to be 

 one fifty-fifth part of the diameter of the aperture of the lens. 

 If the aberration, from unequal refrangibility in a compound 

 object -glafs, vitiates the diftinctnefs lefs than in a fimple object- 

 glafs, in the proportion of one to fix, it may feem a reafonable 

 conclufion, that the leaft circular fpace within which parallel 

 rays of all kinds can be gathered by an object-glafs compofed 

 Vol. III. H of 



