( 698 > 
Poftfcript. . 
JUJi ufon the ckfe of the adjojned Letter ^ I ncehedfrom Mr, 
Gdi^c(ymyyours of May the fourth, ; veherew you are pleafed 
to favour us with aff exa9 ac fount of the famous Experimeftt of 
the coloured Sp^&rum^ lately exhibited he fopet he KoydX Society. 
1 was much rejoycedto fee the trials 0f thdt lllujirious Comfany, 
agree fo exaSly vpith ours here ^ though, in fomewhat ours difagree 
from fi^rMev/totiy as you mil underflar^d hy the iwlofed impar- 
tial acmpt fropf^ 
Sir, Sec. 
Mr, Newton'i Anfmr to the precedent Letter , font to the 
FMfier. 
Sir^ 
'He things oppofed byMr.Line being upon Trials fouDd 
true and granted nie ; J begin with the nm qcef^ioa 
about theproportion of the length of the. Image to its breadth. 
ThiiS I call a ; far,though Mr.L/>?^ in his laft Letter 
fpakeagainfl: fo greOfta length as I affign , yet, as it feems ta 
me^it was not to grant any tranfverfe length fhorter than that 
afllgned by rae^ (for in his firft Letter he abfolutely deaied 
that ch^re wpuld be any. fuch length ;) but to lay the greater 
etpphafis upon his difcQUrfe whi 1ft in defence of common Op- 
'^iqucshe was difputipg in general againft a tranfverfe Image : 
^nd therefore in ;i)y Anfwer I did not prefcribe the juft quan- 
tity of the re/rafting Angle with which I would have the 
Experimem repeated : wi^icb would have been a neceflTary., 
circum£iancc , had the difpute been about the 
"^inm pft mer j^fl proportion of the lengch to the breadth/ 
I added ^ this Note, that the bigger the 
angle of the Prifm is , the greater will be the 
length in proportion to the breadth ; not imagining but that 
when be had found ip^any Prifm the length of the Image tranf- 
verfe to iheaxiS;,; wQuld eafily thence conclude, that a Prifm 
wicha greater angle would make the Image longer, and con=- 
fequently that by ufing an angle great enough he might bring it 
to equal or exceed the lengHi affigncd by me 5 as indeed he 
ssiighi I for, by taking ,^0. Angle of 70 or 75 degrees, or a little 
gireater,, 
