Mr Herschel on the Tints developed by Polarised Light. SS5 
wise. In a paper recently presented to the Royal Society, I 
have shewn that the axes of double refraction in one and the 
same crystal differ in their position according to the colour of the 
intromitted ray, a violet ray being separated into two pencils, 
when incident in the same direction in which a red one would 
be refracted singly. This remarkable fact, which is almost uni- 
versal in crystals with two axes, places the question in a very 
different light. It appears that the nature of the ray, as well as 
that of the medium, has its share in determining the position 
of the axes, and that the intensity of the action of the medium 
on the ray is one of the elements involved in this problem. 
Now, it is hardly possible to conceive the neutral axis of a crys- 
tal otherwise than as a position of equilibrium, or direction in 
which the axis of translation of a luminous molecule (if such 
exist) must be placed, that certain forces may act in opposition, 
and balance one another ; but since forces which balance will 
likewise counteract each other when increased or diminished all 
in the same ratio, it follows that the partial or elementary 
forces so held in equilibrium do not observe the law of propor- 
tionality, when the colour of the incident ray varies. If we sup- 
pose, then, with Dr Brewster, that these partial forces emanate 
from certain fixed axes coincident with remarkable lines in the 
primitive form, it will follow that each separate axis has a pecu- 
liar specific law, which regulates the intensity of its action on the 
differently coloured rays, and that each axis, supposing the 
others not to interfere with it, would exhibit separately a set of 
circular rings, of which the tints would manifest a more or less 
marked deviation from the Newtonian Scale of Colours, as dis- 
played by their uncrystallized laminae. ^ 
This view of the subject will be remarkably supported by the 
facts about to be described, by which it will appear, that among 
crystals with one axis only, there exists the greatest, I might al- 
most say the most capricious diversity in this respect, and that 
probably no two crystals, either with one or two axes, have the 
same scale of action, or polarize the differently coloured rays 
with an energy varying according to the same law precisely. 
To this it may be objected, that from the result of a most ela- 
borate examination of the colours exhibited by sulphate of lime, 
rock-crystal, and mica, M. Biot has concluded that they follow 
