AND ON THE COLOURS EXHIBITED BY CERTAIN FLAMES. 447 



in strong lights, a sensible quantity of yellow, and even a 

 feeble trace of green, might be perceived. If we conceive the 

 abscissa RV, Plate XXVIII. Fig. 1. to represent the prisma- 

 tic soectrum, and erect an ordinate at every point, propor- 

 tional to the number of rays, out of any given quantity, of the 

 corresponding refrangibility the medium is capable of trans- 

 mitting, its extremity will have for its locus a curve of the 

 form royg, which, for brevity's sake, I will call the type of 

 this medium ; the type of a colourless medium being the 

 straight line MN, parallel to the abscissa. 



4. Let t be the thickness of any homogeneous medium, or 

 the number of colorific molecules, of equal absorbent powers, 

 traversed by a ray in its passage through it ; and let C repre- 

 sent the intensity of a ray of any given refrangibility, at its 

 first intromission, y being the ratio of its intensity, after tra- 

 versing a thickness equal to unity, to its original intensity C. 

 Then will 



C + C + C" + &c, or S { C } 



from one end of the spectrum to the other, represent the tint 

 and intensity of the ray at its intromission, while its charac- 

 ter, after traversing the thickness t, will be 



Cy' + Cr/' + C"y"< + &c, = S { Cy« }. 



It would appear, at first sight, that the effect of doubling or 

 tripling the thickness of any coloured medium, would simply 

 be to increase the depth and intensity of the tint, but not to 

 alter its character. If a white object appear blue through a 

 blue glass, we should expect it to appear still bluer through 

 two, and yet more so through three such glasses. The above 

 formula shews, however, that this is so far from being the case, 



vol. ix. p. n. 3 L that 



