Mr. Ifaac Newtonv ConJiderAtioHs on the firmer Reply ; together 
with further DireSiions^ horo to make the Experiments controvert 
ted aright: Written to the PMifher from Cambridge, Novemb. 1 3; 
1675. 
SIR, 
1l \ 7^^" fliew'd me Mr. Line's fecond Letter, I remember 
VV Itoldyou^thatlthoughtan anfvver in writing would be 
infignificant, becaufe the difpute was not about any Ratiocinationj 
but my veracity in relating an Experiment^ which he denies will 
fucceed as it is defer i bed in my printed Letters : For this is ta be 
decided not by difcourfe, but new tryal of the Experiment.Wbat 
it is that impofes upon Mr. Line I cannot imagin; but I fiifpeahe 
has not trycd the Experiment fince he acquainted himftlf with my 
Theory, but depends upon his old notions taken up before he had 
any hint given to obferve the figure of the coloured Image, I fliall 
defire him therefore, before he returns any aufwer^ to try it once 
more for his fatisfaftion, and that according to this manner. 
Lef him take any Prifme, and hold it fo that its Axis may be 
perpendicular to the Sun's rays, and in this pofturc let it be placed 
as clofc as may be to the hole through which the Sun fliines into a 
dark room, which hole may be about the bignefsof a Peafe. Then 
let him turn the Prifm flowly about its Axis, and he fljall fee the 
colours move upon the oppofite wall firft towards that place to 
which the Sun's direft light would pafs, if the Prifm were taken 
away, and then back again. When they arc in the middle of chefe 
two contrary motions, that is,when they are neareft that place to 
which the Sun's direft ray tends, there let him ftop 5 for then are 
the rays equally refrafled on both fides the Prifm. In this pofture 
of the prifm let him obferve the figure of the colours, and he fhall 
find it not round ashe contends,butoblong,andforouch themore 
oblong as the Angle of the Prifm, comprehended by the refrafting 
plains, is bigger, and the wall, on which the colours are cafl:,more 
diftant from the Prifm 5 the colours red, yellow, green, blew,pur- 
pie, fucceed ing in order not from one fide of the figure to the o- 
ther, as in Mr.L/W*s con;efture,but from one end to the other; and 
the length of the Figure being not parallel but tuanverfe to the 
Axis of the Prifm, After this manner I ufed to try the Experi- 
T 1 1 2 men: 
