6 On the UNEQUAL 



ftrument, would be the necefTary confequence of fuch important 

 fteps, towards the perfection of its theory. Thefe expectations 

 have not hitherto been fully anfwered. 



If the theory of the achromatic telefcope is fo complete as it 

 has been reprefented, may it not reafonably be demanded, 

 whence it proceeds, that Huge ni us and others could execute 

 telefcopes with fingle object glafTes eight inches and upwards in 

 diameter, while a compound object glafs of half thefe dimen- 

 fions, is hardly to be met with ? or how it can arife from any 

 defect 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 ? 



The 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. 



It 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- 

 stigation I now do myfelf the honour of fubmitting to the 

 Royal Society. 



The 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 lalt imperfection is fo conftant an at- 

 tendant upon flint-glafs, and every other kind of glafs which 

 poffeiTes the difperfive quality in a confiderable degree, that it 

 has been fufpected, not without appearance of reafon, to arife 

 necefTarily from that ingredient in its compofition on which 



this 



