of Light mthin Solid and Fluid Bodies. 407 



other rays of the same refrangibility are either less dispersible, 

 or apparently indispersible, by the same medium ; but the 

 fact will appear less surprising and anomalous when we advert 

 to certain phaenomena of absorption in which the same pro- 

 perty is displayed. 



The difference between the absorption and the internal 

 dispersion of light is simply this. In the one case the portion 

 of light withdrawn from the intromitted beam is cxtingtiished 

 and invisible, and in the other dispersed and visible ; and we 

 may compare the two classes of phcenomena, by supposing that 

 the light extinguished by absorption is rendered visible as if by 

 dispersion. Now it is a remarkable fact, that almost the whole 

 of the blue light absorbed by the mineral called native orpi- 

 ment 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 dif- 

 ference between the two phaenomena. It would show that 

 the blue light was derived chiefly from the violet, indigo, and 

 blue spaces, and but partially from the green, yellow, oraiige 

 and red, having appropriated the whole of the more refran- 

 gible rays, and but a very small portion of the less refrangible 

 ones ; whereas the blue light from the quiniferous solution is 

 derived almost in equal proportions from all the coloured 

 spaces excepting the least refrangible, red. The limitation 

 of the rays capable of absorption, like the limitation of the 

 dispersible rays in tiie quiniferous solution, is shown in the 

 action of various bodies on the spectrum. Such bodies change 

 the colour of certain spaces in the spectrum, without continuing 

 to absorb the residual rays ; that is, when the absorbable rays 

 are removed by a certain thickness of the body, an additional 

 thickness operates very feebly, as in the quiniferous solution, 

 in altering the colour of the residual beam. 



I have pointed out these analogies between the phajnomena 

 of absorption and dispersion to meet the case of the bright 

 blue line in tiie quiniferous solutions. The dispersion of fluor- 

 spar, and of the glasses and vegetable solutions already de- 

 scribed, is of a iliflerent character. In fluor-spar the dispersion 

 eflected by the first stratum is by no means very abundant; 

 and the intromitted beam, even after passing through one or 



