REFRANGIBILITY of LIGHT. 57 
' From infpeéting the tables of the lengths and apertures of 
telefcopes with fimple objeét-glaffes, 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 ftandard 
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 objedt-glafs will only exceed 
it in length between five and fix times as before. 
Tue 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-glaffes, 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 Newron, 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 diftinétnefs 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 objett-glafs compofed 
Vor. III. H ’ of 
