IN THE INTERIOR OF TRANSPARENT BODIES. 399 



(13°) I have calculated the amount of this dispersion, and shewn 

 how we may determine experimentally whether the vibrations of light 

 are cycloidal or not. (See § 38.) 



(14°) I have shewn that the variation of the velocity of propagation 

 may lead to the formation of dark lines in the spectrum, but I have 

 not attempted to pursue this subject into detail in the present paper. 



(15°) In the course of the paper I have introduced the hypothesis 

 of symmetrical arrangement, merely for the purpose of avoiding com- 

 plexity, but at the conclusion of the paper I have abandoned this 

 hypothesis, and shewn, I think, by simple reasoning, that the results 

 previously obtained on the supposition of symmetrical arrangement are 

 true when the arrangement is not symmetrical, in consequence of the 

 action of the material particles on the etherial. This part of the sub- 

 ject is of considerable importance, as I hope to shew fully hereafter, and 

 most probably leads to an explanation of the absorption of light by 

 transparent bodies, and the natural colours of bodies. (See § 40, &c.) 



All these results, so far as I am aware, are new, except the seventh, 

 which the late Mr Green announced in the Cambridge Philosophical 

 Transactions, Vol. VII. Part i., without, however, proving it. Since 

 I read the present paper before the Society, I have been informed that 

 M. Cauchy has arrived at the same result. 



§ 3. I must here notice a Memoir by M. G. Lame, in the Journal 

 de VEcole Poly technique, Tome XIV, in which his object is to in- 

 vestigate the effect produced by the action of the material par- 

 ticles on the undulations of the ether considered as a continuous fluid 

 mass. The analysis he makes use of is extremely complicated, and 

 is altogether different from that employed in the present paper. 

 There is one of his results which appears to be the same as one of mine, 

 but really is not: it is this, that the force exercised by the particles 

 of matter on the ether produces an alteration in the velocity of light, 

 which gives rise to dispersion: he obtains this result on the express 

 condition, that the force exerted by the particles of matter varies as 

 (distance) ~". Now I have shewn (if my reasoning be correct) that 



Y Y 2 



