414 * Brewster on new Properties of light exhibited 
Hitherto we have avoided all consideration of the cause 
which separates the extraordinary pencils into their component 
colours, nor can we pretend to afford even a plausible con- 
jecture respecting their origin. It is quite obvious that the 
separation into colours is produced before the pencil suffers 
extraordinary reflection, and as the transmitted colours are not 
complementary to those which are reflected, it is equally 
manifest that the phenomenon has no connection with the 
colours of thin plates. If the spectra were produced by the 
ordinary dispersive force of the body, then the dispersion ought 
to be least in gum Arabic, greater in wax, and still greater in 
realgar and the metals, whereas in all these cases the quantity . 
of colour appears to be the same. The extraordinary spectra 
have no resemblance whatever to those which are the effect 
of inflexion ; and even if we could suppose that the light was 
inflected by the grooves, the cause would be inadequate to 
explain the continuance of the colour when the plane of inci- 
dence coincides with the direction of the grooves, and when 
there are manifestly no angles to bend the passing light. 
But whatever be the cause of the phenomena of mother of 
pearl, the facts themselves are peculiarly instructive, and 
naturally lead us to the following conclusions. 
1. Besides the ordinary forces which reflect and refract 
light, there reside without the surface of mother of pearl, and 
of all bodies to which its superficial configuration can be 
imparted, new forces which reflect light, and separate it into 
its component colours. 
2. The lines which bound the space of reflecting activity in 
all surfaces which possess this configuration, are straight, and 
are not parallel to the grooved structure of the surface. Hence 
a surface which appears, even to the unassisted eye, to be full 
