Cdpurrr A^nlin order tl)ereto having darkened my chamber.and 
made a fm^U hole in my wiadow-lhucs , to let in a convenient 
quantity of the Suns iighr, I placed my Prifme at his entraace.that 
It might be thereby retra<!ted to the oppolice wall. It was a: firft 
a very pleafingdivertifcmenfj to view the vivid and inrenfe co- 
lours produced thereby 5 but after a while applying my fdf :o con- 
fider them more circumfpedily, 1 became furprifcd to fee them in 
art oblong fovm'-i which, according to the received laws of Refra- 
ftionj 1 expedted fliould have been circular^ 
They were terminated at the fides with ftreight lines, but at the 
ends, the decay of light was (o gradual, that it was difficult to de- 
termine juftly, what was their figure; yet they fcemed femicir- 
Comparing the length of this coloured SpeUrum with its breadth, 
I found it about five times greater^ a difpropartion fo extrava? 
gant^ that it excited me to a more then ordinary curiofity of ex- 
amining, from whence it might proceed^ I could fcarce think, 
that the various Thickriefs of the glafs, or the termination with flu= 
dow or darknefs, could have any Influence on light to produce 
fuch an effect: 5 yet I thought it not amifs, firft to examine thofe 
circumftanccs, and fo tryed, what would happen by tranfmitting 
iight through parts of the glafs of divers thicknefTes, or through 
holes in the window of divers bigneffes, or by fetting the Prifme 
without fo, that the light might pais through it, and be refrafted 
before it was terminated by the hole ; But I found none of thofe 
circumftances material The faCbion of the colours was in all thefe 
cafes the fame. 
Then I fufpedled, whether by any unevemefs in the glafs, or o- 
ther contingent irregularity, thefe coloi rs might be thus dilated. 
And to try this, J took another Prifme like the former^ and fo 
placed ir, that the light, pafliog through them both^ might be re* 
fra(fied contrary ways, and fo by the latter returned into that 
courfe,from which the former had diverted it. For, by this means 
I thoughr^the regular cfiecSls of the firft Prifme would be deftroyed 
by the fccond Prifme, but the irregular ones more augmented, by 
the multiplicity of refraifdons-» The event was, that the lighr^ 
which by the firft Prifme was difTufed into an oi/c/^g form, was by 
the fecond reduced into ao orbicular one with as much regularity, 
as when it did not at all pafs through them* So that, what ever was 
the caufe of that lengch/cwas not any contingent irregularity. 
