REFRJNGlBILirr of LIGHT. 7 



this quality depends. It is certain that great labour and ex- 

 pence have been beftowed on this object without the defired 

 effect. 



Considering therefore that it is not impoffible to introduce 

 a fluid medium to fupply the place of one of the lenfes, in the 

 compound achromatic object glafs, I was defirous of fearching 

 whether nature afforded fluids poffeffed of the requifite qua- 

 lities. 



It appears from the paffage already quoted, that Sir Isaac 

 Newton not only fufpected that optical inftruments might 

 admit of improvement by a combination of folid and fluid me- 

 diums, but had actually made experiments on the fubject, and 

 confidered this as the mod likely means of carrying thefe inftru- 

 ments to their greateft perfection. 



Dr David Gregory, Savilian profeflbr of aftronomy at 

 Oxford, entertained fimilar ideas on this fubject, as appears 

 from his treatife, entitled, " Catoptrics et Dioptricse Sphsericse 

 " Elementa." In this work, which was publiftied at Edinburgh 

 in the year 1713, he treats of optical inftruments, both by re- 

 fraction and reflection ; and, after (hewing the advantages of 

 the latter in theory, concludes his treatife with the following 

 words : " Quod ft ob difHcultates phyficas, in fpeculis idoneis 

 torno elaborandis et poliendis, etiamnum lentibus uti opor- 

 teat, fortaflis media diverfae denfitatis ad lentem objectivam 

 componendam adhibere utile foret, ut a natura factum ob- 

 fervemus in oculi fabrica, ubi chriftallinus humor (fere ejuf- 

 dem cum vitro virtutis ad radios lucis refringendos) aqueo et 

 vitreo (aquae, quoad refractionem, haud abfimilibus) conjun- 

 gitur, ad imaginem quam diftincte fieri potuit, a natura ni- 

 ** hil fruftra molienti, in oculi fundo depingendam : fed et 

 ' alii funt in animalis oculo, praedicti artificii ufus, qui non 

 " funt hujus loci." 



This coincidence of opinion of thefe great opticians refpect- 

 ing the ultimate perfection attainable by the telefcope, deferves 



to 



(C 



l( 



