S*4 Coloured Image made ly the Prifm. [Book III, 



upon a fheet of white paper, M N, placed at the op 

 polite wall of the chamber, and obferved the figure 

 and dimenfions of the folar image, P T, formed on 

 the paper by that light. This image was oblong, and 

 not oval, but terminated by two rectilinear and pa- 

 rallel fides and two femicircular ends. On its fides it 

 was bounded pretty diftinctly j but on its ends very 

 confufedly and indiftinclly, the light there decaying 

 and vaniihing by degrees. At the diftance of i8| 

 feet from the prifm, the breadth of the image was 

 about 2f inches, but its length was about lo-J inches, 

 and the length of its rectilinear fides about eight 

 inches j and A C B, the refracting angle of the prifm., 

 whereby fo great a length was made, was 64 degrees. 

 With a lefs angle the length of the image was lefs, 

 the breadth remaining the fame. It is farther to be 

 obferved, that the rays went on in ftrait lines from 

 the prifm to the image, and therefore at their going 

 put of the prifm, had all that inclination to one 

 another from which rhe length of the image proceed- 

 ed. This image P T was coloured, and the more 

 eminent colours iay in this order from the bottom at 

 T to t e top at P j red, orange, yellow, green, blue, 

 ind go, violet, together with all their intermediate de- 

 grees, in a continual fuccefiion, perpetually varying.' 



Our philofopher continued his experiments, and by 

 making the rays thus decompounded pafs through a 

 fecond prifm, he found that they did not admit of far- 

 ther decompofition, and that objects placed in the 

 rays producing one colour always appeared to be of 

 that colour. He then examined the ratio between the 

 fin -o of incidence and refraction of thefe decompound- 

 ed rays, and found that each of the feven primary co- 

 lour-making rays, as they may be called, had certain 

 limits wkfiin which they were confined. Thus, let 



