A NEW THEORY OF LIGHT AND COLOURS. 461 



A NEW THEORY OF LIGHT AND COLOURS.* 



By Sir ISAAC NEWTON. 



SIR, — To perform my late promise to you, I shall without further 

 eeremouy acquaint you, that in the beginning of the year 1666 (at 

 which time I applied myself to the grinding of optic glasses of other 

 figures than spherical,) I procured a triangular glass prism, to try 

 therewith the celebrated phenomena of colours. And for that pur- 

 pose having darkened my chamber, and made a small hole in my win- 

 dow shuts, to let in a convenient quantity of the sun's light, I placed 

 my prism at his entrance, that it might be thereby refracted to the 

 opposite wall. It was at first a very pleasing diversion to view the 

 vivid and intense colours produced thereby ; but after a while applying 

 myself to consider them more circumspectly, I was surprised to see them 

 in an oblong form ; which according to the received laws of refraction, I 

 expected would have been circular. They were terminated at the sides 

 with strait lines, but at the ends, the decay of light was so gradual, that 

 it was difficult to determine justly what was their figure; yet they 

 seemed semicircular. 



Comparing the length of this coloured spectrum with its breadth, 

 I found it about five times greater; a disproportion so extravagant, 

 that it excited me to a more than ordinary curiosity of examining from 

 whence it might proceed. I could scarce think, that the various 

 thickness of the glass, or the termination with shadow or darkness, 

 could have any influence on light to produce such an effect; yet I 

 thought it not amiss, first to examine those circumstances, and so 

 tried what would happen by transmitting light through parts of the 

 glass of divers thicknesses, or through holes in the window of divers 

 sizes, or by setting the prism without, so that the light might pass 

 through it, and be refracted before it was terminated by the hole: 

 but I found none of those circumstances material. The fashion of the 

 colours was in all these cases the same. 



Then I suspected, whether by any unevenness in the glass ; or 

 other contingent irregularity, these colours might be thus dilated. 

 And to try this, I took another prism like the former, and so placed 

 it, that the light passing through them both, might be refracted con- 



* This letter communicated to the editor of the transactions of the Royal 

 Society on February 6, 1672, contains the first statement of Newton's discovery 

 of the different colors and refrangibility of the rays of light. 



