PHENOMENA OF ABSORPTION, AND THE COLOURS OF THIN PLATES. 251 
permit lateral transmissions and thus produce compound tints, such as we actually 
observe in natural bodies, and as we have shown to be produced by thin plates. 
Now if we suppose the colouring particles to be spherical, or to have the form of 
plates or cubes, or other solids disseminated through the fluid or solid bodies which 
they colour, the tints would be permanent and compound as we find them in nature. 
The second point of difference to which I have referred, namely the absolute dis- 
appearance of the reflected tints in several coloured solids, fluids, and gases, is one 
of great magnitude. Newton has evaded this difficulty in his theory ; but from the 
manner in which he gets rid of the intromitted light in black bodies, it is obvious 
that he would ascribe the disappearance of the reflected tints to their being “ va- 
riously reflected to and fro until they happened to be stifled and lost.” 
As I shall have occasion to discuss this subject experimentally in a paper on the 
permanent colours of natural bodies, I shall only state at present that 1 have suc- 
ceeded by particular methods in rendering reflected tints visible in many coloured 
fluids and glasses, but I cannot consider them as equivalent to the reflexions of thin 
plates. 
I have endeavoured to corroborate the views contained in the preceding pages by 
a series of collateral experiments on the periodical colours of polarized light. When 
we divide the spectrum into bands by doubly refracting plates, the phenomena are 
beautiful beyond all description. If we dissect or subdivide the luminous bands in 
the spectrum, as seen by one analysing prism, by means of successive plates and 
prisms, the result is very remarkable ; and if the doubly refracting plates are inclined 
to each other or to the incident beam, the black bands will also be inclined to each 
other, and the luminous spaces have the form of a triangle either complete or trun- 
cated at its apex. By using plates of the same or of various substances*, and placing 
their axes in different azimuths to the plane of primitive polarization, we obtain ex- 
tremely singular spectra, in which the bands approximate to those of absorbing 
media. 
But there is another result of this class of experiments to which I would especially 
call the attention of philosophers. The colours of the bands thus produced have no 
* I have constructed apparatuses of this kind made out of composite crystals of calcareous spar, including 
one and more thin plates of its own substance. The beautiful and apparently capricious tints which such 
crystals exhibit when properly cut into prisms, or when prisms are applied to their surface, are nothing more 
than the luminous bands of the spectrum subdivided by one or more dissections. I have now before me such a 
crystal, in which a prism cemented externally brings out the spectrum, which would otherwise have suffered 
total internal reflexion. A virtual prism forming part of the rhomb polarizes the incident light, an included 
hemitrope plate affords the polarized tints, and a second virtual prism analyses the light which the plate trans- 
mits. In some parts of the rhomb there are plates of different thickness, by which the luminous hands are 
beautifully subdivided. In this manner by the slight aid of an applied prism we are furnished with a com- 
plicated optical apparatus. Such a combination, which it is easy to make artificially by inclosing thin doubly 
refracting plates between prisms of calcareous spar, affords an ocular explanation of those beautiful forms 
of the system of polarized rings which are produced in composite crystals of calcareous spar. These subdivided 
bands, indeed, are portions of that system seen obliquely by prismatic refraction. 
MDCCCXXXVII. 2 L 
