Absorption and the Colours of TJiin Plates. 209 



In arriving at these conclusions, however, and drawing a 

 distinct line between the phaenomena of absorption and those 

 of thin plates, two classes of facts are compared under very 

 different circumstances. In the one case philosophers have 

 studied in cumulo the result of the successive actions of an 

 infinite number of the colorific particles upon the intromitted 

 light, whereas in the other case they have observed only the 

 colour of a single particle, whose thickness is equal to that of 

 the films of air, water, glass and mica submitted to experi- 

 ment. The impracticability of combining a number of such 

 films, and studying their united action upon light, was doubt- 

 less the reason which prevented natural philosophers from 

 bringing the two series of facts under the same conditions. 

 Sir Isaac Newton, indeed, had spoken so confidently of the 

 result of such a combination, as to discourage any attempts 

 to effect it ; and it is a singular fact that his successors have 

 never called in question his bold though ingenious assump- 

 tion. " If a thinned or plated body," says he, " which being 

 of an even thickness, appears all over of an uniform colour, 

 shall be slit into threads or broken into fragments of the same 

 thickness with the plate, I see no reason why every thread or 

 fragment should not keep its colour, and by consequence why 

 a heap of those threads or fragments should not constitute a 

 mass or powder of the same colour which the plate exhibited 

 before it was broken. And the parts of all natural bodies 

 being like so many fragments of a plate, must on the same 

 grounds exhibit the same colours." 



This remarkable opinion I have often been desirous to sub- 

 mit to the test of direct experiment, in the conviction that the 

 result would be different from what is here stated ; but I have 

 been baffled in every attempt to make such an experiment ; 

 and had not accidental circumstances placed in my hands two 

 substances in which thin plates were combined nearly in the 

 very manner which I wished, and which I believe had never 

 before been submitted to examination, the problem might 

 have remained long without a solution. 



The first of these substances to which my attention was 

 called, is the remarkable nacreous body which Mr. Horner 

 has described in the last volume of the Transactions, and 

 whose singular optical properties I have explained in a letter 

 which accompanies his paper. This substance consists of 

 laminae of considerable transparency, separated by extremely 

 thin films, which exhibit in the most brilliant manner the co- 

 lours of thin plates. 



In order to compare the effect produced by a number of 

 such films with that of a single film, we must either analyse 



Phil. Mag. S. 3. Vol. 21 . No. 1 37. Sept. 1 84-2. P 



