(499) 
A Letter of Mn Franc. Linus, vpritten to the Puhlifher from Liege 
the 2$th of Febr. 1675. ft'i^« l^^^^g ^ R^P^f ^0 Letter printed 
in ]>\mhAio* hyrvay of Anfwerto a former Letter of the fame 
Mr. Linus, comermng Mnlfaac Newton's Theory of Light mi 
Colours. 
Homured Sir^ 
IN yours of Dec.i'j.v^hxch I received about the end ofJan^oM 
fay,! may reft alTured, Firjl^ that the Experiment was made in 
clear days. Seco^dly^ that the Prifm was placed clofe to the ho!e^ 
fo that the light had no room to diverge : And thirdly^ that the I- 
mage was not Parallel (as I conjedured ) but Tranfverre to the 
Axisof the Prifm. Truly, Sir, if thefe AflTertions be admiciedj 
they do indeed dircdly cut off whit I faid of Mr. iVifiyWs being 
deceived by a bright cloud. Bat if we compare them with Mr. 
Nev0ton\ Relation of the Experiment in the Phil,1ranf<i^ionSyN'. 
80/^.3076. it will evidently appear, they cannot be admitted as 
being direftly contrary to what is there delivered. For there he 
tells us, the ends of the coloured Image, he faw on the oppofic 
wall, near five times as long as broad, y^ewe*^ to be Semicircular^ 
Novv thefe Semicircular Ends are never feen in a clear day, as Ex- 
perience fhews. From whence follows againft the firjl kfTenion^ 
That the Experiment was not made in a clear day,Neither are thofe 
Semicircular Ends ever feen, when the Prifin is placed clofe to 
the Hole; which contradifts the feco^d Affertion. Neither are 
they ever feen, when the Image is Tranfverfe to the length or Axis 
of the Prifm ; which direftly oppofes the third AfTertion. But if 
in any of thefe three Cafes, the Image be made fo much longer than 
broad (as eafily it may,by turning the Prifm a little about its Ax- 
is) near five times as long as broad,than the one End thereof will run 
out into a fliarp Cone or Pyramis like the flame of a Candle,and the 
other into a Cone fomewhat more blunt ; both which are far from 
feeming Semicircular : Whereas, if the Image be made not in a 
clear day ,but with a bright cloud^and the Prifm not placed clofc 
to the Hole^ but in a competent diftance from the fame (as you fee 
it placed in the Scheme of the Experiment in i\7.S4. 4091.J 
then thefe Semicircular Ends always appear with the fides there- 
of ftraight lines ju ft as Mr. i\7w^(>;^ there defer ibes them. Neither 
T t t is 
