6 2 On the UNEQUAL 
{trument, would be the neceffary confequence of fuch important 
fteps, towards the perfection of its theory. Thefe expectations 
have not hitherto been fully anfwered. } 
Ir the theory of the achromatic telefcope is fo complete as it 
has been reprefented, may it not reafonably be demanded, 
whence it proceeds, that HuceNivus and others could execute 
telefcopes with fingle object glaffes eight inches and upwards in 
diameter, while a compound objet glafs of half thefe dimen- 
fions, is hardly to be met with? or how it can arife from any 
defe& in the execution, that reflectors can be made fo much 
fhorter than achromatic refractors of equal apertures, when it 
is well known that the latter are much lefs affected by any im- 
perfections in the execution of the lenfes compofing the object 
glafs, than reflectors are by equal defects in the figure of the 
great fpeculum ? / 
Tue general anfwer made by artifts to enquiries of this kind, 
is, that the fault lies in the imperfection of glafs, arid particu- 
larly in that kind of glafs of which the concave lens of the 
compound object glafs is formed, called flint-glafs. 
Ir was in order to fatisfy myfelf concerning the reality of 
this difficulty, and to attempt to remove it, that I engaged in 
the following courfe of experiments. The refult of this inve- 
ftigation I now do myfelf the honour of fubmitting to the 
Royal Society. 
Tue imperfections of glafs for optical purpofes arife partly 
from its want of perfect tranfparency, and from being more or 
lefs affected with a tinge of fome particular colour, but princi- 
pally from irregularities which are frequently found in its re- 
fractive denfity. This laft imperfection is fo conftant an at- 
tendant upon flint-glafs, and every other kind of glafs which 
poffeffes the difperfive quality in a confiderable degree, that it 
has been fufpected, not without appearance of reafon, to arife 
neceflarily from that ingredient in its compofition on which 
this 
