AND ON THE COLOURS EXHIBITED BY CERTAIN FLAMES. 451 



other words the red, thus insulated, is of too feeble an illumi- 

 nating power to affect the sight in the immediate vicinity of 

 the other more brilliant rays, and only becomes visible when 

 they are extinguished, or greatly enfeebled. To an eye defended 

 by such a glass, vision, through a prism with the largest re- 

 fracting angle, is as sharp, and the outlines of minute objects 

 as free from nebulosity and indistinctness, as if the rays had 

 suffered no refraction. These characters, — the absolute ho- 

 mogeneity of the rays, — their situation precisely at the least 

 refracted limit of the spectrum, and the facility with which 

 they may be insulated, render them of peculiar importance as 

 standards of comparison in optical experiments. I shall in fu- 

 ture always be understood to speak of these, when I use the 

 expression extreme red rays. 



8. Almost analogous in its type to that of Figures 2, 3, 

 and 4., but differing from it in a higher degree of insulation 

 of the yellow rays, is a species common enough in ancient 

 windows. Its tint is a blue, much less vivid than the blue glass 

 above mentioned, and bordering rather on grey or slate co- 

 lour. A thickness 0.184 in. of this glass, is sufficient to pro- 

 duce a very sharp termination of the yellow band on the least 

 refrangible side, and almost to obliterate the green. The yel- 

 low portion of the spectrum is thus placed in the clearest evi- 

 dence. It is of great brightness, and by no means that almost 

 infinitesimal line which Dr Young seems to have regarded it ; 

 its breadth being nearly ith of the interval between the red 

 and blue parts of the spectrum. By the aid of this glass, 

 I have succeeded in insulating a yellow ray of considerable 

 strength, cutting off the more refrangible end of the spectrum 

 by a yellow or light-brown glass, and the less by a bluish-green 

 one, and, though not quite homogeneous, having still an ad- 

 hering 



