246 
SIR DAVID BREWSTER ON THE CONNEXION BETWEEN CERTAIN 
colour of a single particle, whose thickness is equal to that of the films of air, water, 
glass and mica submitted to experiment. The impracticability of combining a number 
of such films, and studying their united action upon light, was doubtless 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 as- 
sumption. “ If a thinned or plated body,” says he, <e which being of an even thick- 
ness, 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 submit 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 Transac- 
tions, and whose singular optical properties I have explained in a letter which accom- 
panies his paper. This substance consists of laminae of considerable transparency, 
separated by extremely thin films, which exhibit in the most brilliant manner the 
colours 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 the light reflected and transmitted by a single 
film by means of a fine prism placed in front of a telescope, or examine the prismatic 
spectrum produced by such an apparatus when it is reflected or transmitted by the 
film in question. When we thus examine the reflected tints of the three first orders 
of colours, we find them to consist of that part of the spectrum which gives the pre- 
dominating colour of the tint mixed with the rays on each side of it. The reflected 
green of the third order, for example, consists of the green part of the spectrum, 
bounded on one side with some blue, and on the other side with some yellow rays, 
all the rest of the spectrum being wanting, having passed, as it were, into the trans- 
mitted beam. In analysing, therefore, the transmitted beam, its spectrum is found to 
consist only of the violet and blue, and the orange and red spaces, a dark band cor- 
responding to the reflected spectrum separating it into two parts. In the higher 
orders of colours the reflected spectrum consists of two or more portions separated 
