176 PHILOSOPHICAL TRANSACTIONS. [aNNO 1675. 



length and breadth of this coloured spectrum ; but never found it so when the 

 sky was clear and free from clouds near the sun ; but then only appeared this 

 difference of length and breadth, when the sun either shined through a white 

 cloud, or enlightened some such clouds near unto it. And then indeed it was 

 no marvel the said spectrum should be longer than broad; since the cloud or 

 clouds so enlightened, were in order to those colours like to a great sun, making 

 a far greater angle of intersection in the said hole, than the true rays of the sun 

 make; and therefore are able to enlighten the whole length of the prism, and 

 not some small part of it only, as we see enlightened by the true sun beams 

 coming through the same little hole. And this we behold also in the true sun 

 beams when they enlighten the whole prism : for although in a clear heaven, 

 the rays of the sun passing through the said hole, never make a spectrum longer 

 than broad, because they then occupy but a small part of the prism; yet if the 

 hole be so much larger as to enlighten the whole prism, you shall presently see 

 the length of the spectrum much exceed its breadth; which excess will be always 

 so much the greater, as the length of the prism exceeds its breadth. From 

 whence I conclude, that the spectrum, this learned author saw much longer 

 than broad, was not effected by the true sun beams, but by rays proceeding 

 from some bright cloud as is said; and by consequence, that the theory of light 

 grounded on that experiment cannot subsist. 



What I have here said needs no other confirmation than mere experience, 

 which any one may quickly try; neither have I only tried the same on this occa- 

 sion, but near 30 years ago showed the same, together with divers other expe- 

 riments of light, to that worthy promoter of experimental philosophy. Sir Kenelm 

 Digby, who coming into these parts to take the spa waters, resorted oftentimes 

 to my darkened chamber, to see those various pha^nomena of light made by 

 divers refractions and reflexions, and took notes upon them ; which industry if 

 they also had used, who endeavour to explicate the aforesaid difference between 

 the length and breadth of this coloured spectrum, by the received laws of re- 

 fraction, would never have taken so impossible a task in hand. 



The rest is, honoured sir, that it is far from my intent, that the mistake here 

 mentioned do any way derogate from that learned person: which truly might 

 have happened to myself, if at my first trial thereof, the sun had been in a 

 white cloud, as it seems, it happened to him. Wherefore ceasing further to 

 trouble you, I rest, yours to command, Francis Linus. 



October 6, l674. 



j4n Answer to this Letter. 

 Sir, — ^The letter you thought fit to write by way of animadversion upon Mr. 



