Mr. B. Stewart on certain Laws of Chromatic Dispersion. 143 



with a little water, were shaken in a stoppered bottle for some 

 time, so as to saturate the ether with hydrochloric acid. When 

 the solution formed by the decolorization of the chlorophyll by 

 bases was shaken with this solution, a remarkable change took 

 place; the ether retained the yellow matter, and the hydro- 

 chloric acid the blue colouring principle. The two colours were 

 thus isolated ; but if now alcohol in excess was added, which dis- 

 solves both the yellow and green colouring matters and their 

 solvents, the solution became of the original green tint. 



To the yellow matter soluble in ether, Fremy gives the name 

 phylloxanthine, and to the blue colouring matter the name phyllo- 

 cyanine. To the other yellow body which results from the change 

 of phyllocyanine, he gives the name phylloxantheine. 



The blue and yellow colouring matters may be obtained 

 directly from chlorophyll by adding the ether and acid mixture 

 directly to the alcoholic extract of the leaves. The green first 

 becomes brown and is then resolved into phyllocyanine, which 

 dissolves in the acid, and phylloxanthine, which dissolves in the 

 ether. This interesting experiment may also be made directly 

 with the leaves. 



Fremy found that the yellow colouring matter formed in the 

 young shoots is the same as that resulting from the decom- 

 position of chlorophyll. It may be extracted by alcohol, and 

 partially resolved into yellow and a little blue colour by means 

 of hydrochloric acid and ether. Leaves which become yellow 

 in autumn, then only contain phylloxanthine. 



XVI. Note regarding Mr. Ponton's Paper, " On certain Laws 

 of Chromatic Dispersion." By Balfour Stewart. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 



HAVING perused a paper " On certain Laws of Chromatic 

 Dispersion" which appeared in your Journal of last March 

 as a communication from Mr. Ponton, I feel the following diffi- 

 culty in recognizing as additions to our knowledge certain laws 

 which this author claims to have discovered. 



To adopt Mr. Ponton's notation, let U be the length in free 

 aether of the undulation corresponding to one of Fraunhofer's 

 seven fixed lines, and let u denote the wave-length of the same 

 line within a given medium after refraction. Then the relation 

 between U and u may be expressed by the equation 



€(u-t-a±cc) = Vj 



where the quantities e and a are constant for the same medium 

 and temperature, being the same for all undulations ; while, on 



