They were .terminated at the fides with ftreight 

 Lines, but at the ends, the decay of Light was 

 fo gradual, that it was difficult to determine juft- 

 ly, what was their Figure , yet they leem'd Se- 

 micircular. 



Comparing the length of this colour\J Spe» 

 Brum with its breadth, I found it" about five times 

 greater ; a difproportion fo extravagant,: that 

 it excited me to a more than ordinary Curiofity 

 of examining, from whence it might proceed. I 

 could fcarce think, that the various thicknels of 

 the Glals, or the termination with (hadow or 

 darkneis, could have any Influence on Light to 

 produce luch an enect ; yet I thought it not 

 amsfs, firft to examine thofe Gircumftances, and 

 fo try'd what would happen by tranfmitting 

 Light through parrs of the Glafs of divers thick- 

 nefles, or through holes in the Window of divers 

 bignefles, or by fetting the Prifm without, fo that 

 the Light might pals through it, and be refra£l> 

 ed before it was terminated by the hole : But I 

 found none of thofe Circumftances material. The 

 faftiion of the Colours was, in all thefe Cafes, 

 the fame. 



Then I mfpe&ed, whether by any uneven- * 

 nefs in the Giafs, or other contingent Irregu- 

 larity, thefe Colours might be thus dilated. And 

 to try this, I took another Prifm like the former, 

 and fo plac'd it, that the Light pafling through 

 them both, might be refracted contrary ways, 

 T and fo by the latter returned into that Courfe, 

 from which the former had diverted it. For, 

 by this means, I thought the regular efFe6h of 

 the firft Prifm would be deftroy'd by the fecond 

 Prifm, but the irregular ones more augmented 

 by the multiplicity of Refractions, The Event 

 was, that the Light, which by the firft Prifm 



was 



