116 SIR DAVID BREWSTER ON THE DECOMPOSITION AND DISPERSION OF LIGHT 



dispersion, provided the thickness of the medium is not so great as to have dis- 

 persed all the dispersible rays. 



2. Wlien such a medium is thus rendered incapable of dispersing more light, 

 it is not because it lias lost a property which it originally possessed, but because 

 it is deprived of all the dispersible rays which it contained. 



It is no doubt an interesting fact, that a small number of differently coloured 

 rays, constituting blue light by their mixture, should possess this property of 

 being dispersed, while other rays of the same refrangibility ai'e either less disper- 

 sible, or apparently indispersible, by the same medium ; but the fact will appear 

 less surprising and anomalous when we advert to certain phenomena of absorp- 

 tion in which the same property is displayed. 



The difference between the ahsorj^tion and the internal dispersion of light is 

 simpl}^ this. In the one case the portion of light withdrawn from the intromitted 

 beam is extinguished and invisible, and in the other dispersed and visible ; and we 

 may compare the two classes of phenomena by supposing that the light extin- 

 guished by absorption is rendered visible as if by dispersion. Now it is a remark- 

 able fact, that almost the whole of the blue hght absorbed by the mineral called 

 native orpiment is extinguished during the passage of the light through the first 

 stratum, whose thickness is less than the fiftieth of an inch ; and hence it is that 

 the thinnest slice of this substance has nearly as deep a yellow colour as the 

 thickest. Were the absorbed blue rays to become visible by dispersion, we should 

 actually see a more striking example of epipolism, or dispersion confined to the 

 first stratum, than in the quiniferous solution. Even the condensation of the 

 beam would not in this case give us a blue cone of light. 



The analysis of the blue line indeed would indicate a difference between the 

 two phenomena. It would shew that the blue light was derived chiefly from the 

 violet, indigo, and blue spaces, and but partially from the green, yelloiv, orange, and 

 red, having appropriated the whole of the more refrangible rays, and but a very 

 small portion of the less refrangible ones ; whereas the blue light from the quini- 

 ferous solution is derived almost in equal proportions from all the coloured spaces 

 excepting tlie least refrangible, red. The limitation of the rays capable of ab- 

 sorption, like the limitation of the dispersible rays in the quiniferous solution, is 

 shewn in the action of various bodies on the spectrum. Such bodies change the 

 colour of certain spaces in the spectrum, without continuing to absorli the resi- 

 dual rays ; that is, when the absorbable rays are removed by a certain thiclcness 

 of the body, an additional thickness operates very feebly, as in the quiniferous so- 

 lution, in altering the colour of the residual beam. 



I have pointed out these analogies between the phenomena of absorption 

 and dispersion to meet the case of the bright blue hue in the quiniferous solu- 

 tions. The dispersion of fluor-spar, and of the glasses and vegetable solutions 

 already described, is of a different character. In fluor-spar the dispersion effected 



