( 3079 ) 
And I faw by the variation of thofe places^that the lighr^ tending 
to that end of the Imagej towards which the refradion of the^firlt 
Prifme was made, did in the fecond Prifme fuffer a Refradion . 
confiderably greater then the h'ght tending to the other end. And 
fo the true caufe of the length of that Image was deteded to b j: 
no otherj then that Li^ht confi&s of l^ys differejidy refrajiojbhj 
which^without any refped to a difference in their incidence^werej 
according to their degrees of refrangibilicy, craafmiited towards 
divers parts of the wall. 
When I underftood this, I left oiTmy aforef iid Glafs works ; 
for I faw^ that the perfcdhon of Teicfcopes was hitherto limited^ 
not fo much for want of glaffcs truly figured according to the pre- 
fcriptioDS of Optick AiiihorSj (which, ail men have hitherto inia- 
gioedj) as becaufe that Light it felfis a HcterooencDu^ mixture of^ 
differ ently refrangible J{ayf, So that, were aglafs lo exafl-iy figured, 
as to colled any one fort of rays into one poip.tg it could no: coh 
left thofe alfo into the fame point, which liaving the fame Inci- 
dence upon the fame Medium are apt to fuff n' a difi^rrent refradi- 
on, Nay^ I wondered, that feeing the diffii^rence of rcfrangibili- 
ty was fo great^ as I found it, Teleicopes fliould arrive to that per^ 
fedioD they are now at* For^meafuring the refraftions in one of 
my Prifmes, I foundg that fuppofing the common JIne of Inci» 
dence upon one of its planes was 44 parts, the//^^ of refradion of 
theutmoft Rays on the red end ofthe Colours, made our of the 
glafs into the Air^ would be 68 parts^ and the Jiiw of refradion of 
the utmoft rays 00 the other end^ 69 parts : So that the difference 
is about a 24^/^ or 2 5/Z>pvirtof the whole refraftion. Andonfe- 
quently, theobjeft-gl^sof any Telefcope cannot colkd all the 
raySj which come from one point of an ©bjt d fo as to make them 
convene at its focus in lefs room then in a circular fpace ^ whofs 
diameter is the 5o^i"p2rtof the Diameter of its Aperture 5 which 
is an irregularity, fome hundreds of times grearer^ then a circu- 
larly figured Lens, of fo fmall a feftion as the Objcd-glaffes of 
long Telefcopes are,would caufe by the unfitnefs of its figure^were 
Light uniform. 
This made me take T\efieUions into coofideration, and finding 
themregularj fo that the Angleof Refleftion of all forts of Pk^vs 
was equal to their Angle of Incidence ; I underftoodgChat by their 
mediation Optick inflruments might be brought to any degree of 
perfedioa imagmable, provided a ReflcUing fubftance could be 
foundj 
