REFRANGIBILITY of LIGHT. " 
this quality depends. It is certain that great labour and ex- 
pence have been beftowed on this object without the defired 
effect. . 
Cons!pERiNG therefore that it is not impoffible to introduce 
2 fluid medium to fupply the place of one of the lenfes, in the 
compound achromatic object glafs, 1 was defirous of fearching 
whether nature afforded fluids poffeffed of the requifite qua- 
lities. - : t 
Ir appears from the paflage 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 atually made experiments on the fubje, and 
confidered this as the moft likely means of carrying thefe inftru- 
ments to their greateft perfection. 
“Dr Davip Grecory, Savilian profeffor of aftronomy at 
Oxford, entertained fimilar ideas on this fubject, as appears 
from his treatife, entitled, ‘‘ Catoptrice et Dioptrice Sphezricz 
“ Elementa.” In this work, which was publifhed at Edinburgh 
in the year 1713, he treats of optical inftruments, both by re- 
fraction and refletion ; and, after fhewing the advantages of 
‘the latter in theory, concludes his treatife with the following 
words: “ Quod fi ob difficultates phyficas, in {peculis idoneis 
“ torno elaborandis et poliendis, etiamnum lentibus uti opor- 
teat, fortaflis media diverfe 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 (aquz, quoad refractionem, haud abfimilibus) conjun- 
gitur, ad imaginem quam diftinéé fieri potuit, a natura mi- 
hil fruftra molienti, in oculi fundo depingendam: fed et 
alii funt in animalis oculo, prediti artificii ufus, qui non 
fant hujus loci.” , ; 
‘Tuts coincidence of opinion of thefe great opticians refpect- 
ing the ultimate perfection attainable by the telefcope, deferves 
; to 
