rOX.. Vt.] PHILOSOPHICAt TRANSACTIONS. 67Q 



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 pro- 

 cured a triangular glass prism, to try therewith the celebrated pha^nomena of 

 colours. And for that purpose having darkened my chamber, and made a 

 small hole in my window 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 con- 

 sider 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 straight 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 set- 

 ting the prism without, so that the light might pass through it, and be re- 

 fracted before it was terminated by the hole : but I found none of those cir- 

 cumstances 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 con- 

 tingent 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 contrary ways, and so by the latter re- 

 turned into that course from which the former had diverted it. For, by this 

 means, I thought the regular effects of the first prism would be destroyed by 

 the second, but the irregular ones more augmented, by the multiplicity of re- 

 fractions. The event was, that the light, which by the first prism was diffused 

 into an oblong form, was by the second reduced into an orbicular one, with as 

 much regularity as when it did not at all pass through them. So that, what- 

 ever was the cause of that length, it was not any contingent irregularity. 



I then proceeded to examine more critically, what might be effected by the 

 difference of the incidence of rays coming from divers parts of the sun ; and to 

 that end measured the several lines and angles, belonging to the image. Its 



