250 
SIR DAVID BREWSTER ON THE CONNEXION BETWEEN CERTAIN 
The reflected tints of course vary with the obliquity of the incident light ; and at 
great incidences the transmitted ones, however splendid and varied, all become pale 
yellow. When these combinations of glass films are immersed in a balsam or an oil, 
their colours whether transmitted or reflected all disappear, excepting a pale yellow 
light like that which is transmitted at great incidences. These facts prove, beyond 
a doubt, that the transmitted colours, though wholly unlike to those of thin plates, 
are yet produced by the same cause and are residuary, and generally complementary 
to the hue of the reflected tints. 
The analysis of these colours by the prism affords a series of most beautiful and 
instructive phenomena, and it is only by coloured drawings that any adequate idea of 
them can be conveyed. All the phenomena of coloured media, with bands of various 
breadths and various intensities of illumination, are exhibited in great perfection, so 
as to identify completely in this feature the two classes of facts. But what is still 
more striking, the colours of the bands are changed, and we thus find that the cha- 
racteristic phenomenon of absorption is produced by the action of thin plates. To 
such a degree indeed is the change of tint carried, that I have insulated a white band 
in the orange part of the spectrum. 
Notwithstanding this identification of absorption and periodical action in their pri- 
mary features, there are two points of difference which separate widely the two classes 
of phenomena. The first of these is, that the bands and tints of absorbing media are 
not changed by obliquity, and the second, that the reflected tints are not visible in 
such media. Sir Isaac Newton endeavoured to remove the first of these difficulties 
by supposing that the particles of bodies on which their colours depended have an 
enormous refractive power ; and M. Biot* has endeavoured to meet it more effectually 
by introducing two new suppositions, viz. that the particles are capable of transmit- 
ting light only through their centre of gravity, and that the lateral transmissions may 
be prevented or turned aside by the inflecting forces which act at a distance on the 
luminous molecules which approach them. 
These explanations of the uniformity of the tints at all incidences have been ren- 
dered necessary, not perhaps by the real difficulties of the case, but in consequence of 
Sir Isaac Newton and his followers taking it for granted that the colours of natural 
bodies were pure tints of a particular order. Hence it becomes a necessary assump- 
tion in the theory that the particles had sizes corresponding to these pure tints, and 
that the light which composed them should not pass through different thicknesses of 
these particles. As I have demonstrated, however, in a paper already referred to, 
that the tint which Newton reckoned one of the third order, has no connexion what- 
ever with that or with any other order, and that all other tints of absorbent media 
are in the same predicament, we are not only free from the difficulty which embar- 
rassed Newton ; but it is actually necessary to have recourse to particles of an ordi- 
nary refractive power, and having such forms and occupying such positions as will 
* Traite de Physique, tom. iv. p. 126 . 
