Dr Brewster on a New Analysis of Solar Light. 201 



curve is every where within the least refrangible branch YM 

 of the yellow curve, and the most refrangible branch RN of 

 the red curve every where within the most refrangible branch 

 YN of the yellow curve. On this supposition the excess of 

 blue light over the yellow will begin to modify the red space 

 at that point where the ordinates ex, bx, are in the ratio of 2 

 to 5 ; the ratio in which they exist in white light, and the ex- 

 cess of red light over the yellow, will begin to modify the blue 

 space at that point where the ordinates of the most refrangible 

 red, and most refrangible yellow, branch are as 3 to 5, the 

 ratio in which the corresponding rays exist in white light. 

 But it is not improbable that the blue branch BM may actu- 

 ally cross the yellow branch YM at some point m, as shown 

 in Fig. 5 ; and the red branch RN the yellow branch YN, so 

 that the blue ordinates in the one case, and the red ordinates 

 in the other, will exceed the yellow ordinates at every point 

 beyond the points of intersection m and n. If this should 

 prove to be true, it follows, that at and beyond n the red 

 should, as it were, re-appear, and by its predominance convert 

 the extreme blue space between n and N into violet. 



In every part of a spectrum thus composed, there neces- 

 sarily exists three different colours, which form a compound 

 tint by their union ; and as the three differently coloured rays 

 have at every point the same refrangibility, it is impossible to 

 separate them, or to analyze the compound tint by prismatic 

 refraction. By transmitting the compound tint, however, 

 through transparent solids or fluids, which absorb one or more; 

 of the simple rays, and allowing the rest to pass, we may ex- 

 hibit one or more of the rays separately, or obtain a residual 

 colour, which indicates the presence of rays whose existence 

 cannot be inferred from the original colour of the compound 

 tint. If, for example, we transmit the compound ray at x, 

 Fig. 4, through an absorbing medium, which detains 27 red 

 rays, we shall obtain a transmitted tint with 11 yellow rays, 

 and 10 of white light, or a brilliant yellow; and if we again 

 transmit this liffht through another medium which absorbs 11 

 yellow rays, we shall have a pure white light, composed of 8 

 red, 5 yellow, and 2 blue rays. This white light will exhibit 

 the singular properly of homogeneous light ; namely, thai of 



