272 Dr. Brewster on the laws of polarisation, &c. 
this class of colours. The various phenomena of thick and 
thin plates ; the colours of inflected light ; the hues of the 
supernumerary rainbow ; the fringes which I observed with 
two inclined plates of glass of equal thickness ; the colours 
noticed by Mr. Nicholson and Mr. Knox, with plates cf 
unequal thickness; the communicable colours of mother 
of pearl and striated surfaces ; and the colours which I 
have discovered by the successive reflection of polarised 
light between metallic plates, and surfaces that produce total 
reflection, are all referable to this simple law. But while we 
thus admire the wide range of phenomena which the law of 
interference embraces, it is necessary also to state, that its 
application to the colours of polarised light is still attended 
with some difficulties, and that there are other phenomena of 
complementary colours which I have lately observed, that 
resist this method of classification. These, however, are 
merely difficulties in the application of the law, and not 
objections to its generality ; and I have no doubt that Dr. 
Young will succeed in referring them to the same cause, and 
Will thus add to the honour which already belongs to him, of 
having generalised a long train of perplexing and important 
phenomena.* 
I have the honour to be, &c. &c. 
DAVID BREWSTER. 
To the Right Hon. 
Sir Joseph Banks , Bart. &c. &c. &c. 
* Having communicated to Dr. Young the contents of the preceding paper, 
he has requested me to subjoin to it the following letter : 
“ My Dear Sir, 
,e Your experiments, on the colours afforded by crystals having two 
optical axes, appear to establish a very important result in the theory of light ; for 
