Intelligence and Miscellaneous Articles. 477 
November 26.—Professor Challis made a communication “‘ On the 
Solar Eclipse of July 18, 1860.” 
December 10.—Mr. Seeley read a “‘ Notice of Opinions on the 
Red Limestone at Hunstanton.” 
Professor Miller also described “ An Instrument for measuring the 
radii of arcs of Rainbows.” 
February 11, 1861.—Mr. H. D. Macleod read a paper ‘‘ On the 
present State of the Science of Political Economy.” , 
The writer took a general survey of the science as it at present 
exists, testing several generally received doctrines by the principles 
of inductive logic, and earnestly enforcing the necessity of a thorough 
reform of the whole science, which must be constructed on prin- 
ciples analogous to those of the other inductive sciences. 
February 25.—Dr. Humphry made a communication ‘On the 
Growth of Bones.” } 
March 11.—The Master of Trinity made a communication ‘‘ On 
the Timzeus of Plato.” 
LXX. Intelligence and Miscellaneous Articles. 
ON THE OPTICAL PROPERTIES OF THE PICRATE OF MANGANESE. 
BY M. CAREY LEA. 
So and Haidinger have described a remarkable property 
possessed by certain crystalline surfaces, of reflecting, besides 
the ray normally polarized in the plane of incidence and reflexion, 
another ray, polarized perpendicularly to that plane, and differing 
from the former in being coloured—a property rendered more con- 
spicuous by the fact that the colour of the ray so polarized abnormally 
is either complementary to, or at least quite distinct from the colour of 
the crystal itself. 
I find that this property is possessed to a remarkable degree by the 
picrate of manganese. ‘This salt crystallizes in large and beautiful 
_ transparent right-rhombic prisms, sometimes amber-yellow, some- 
times aurora-red, exhibiting generally the combination of principal 
. prism, and macrodiagonal, brachydiagonal and principal end planes. 
In describing this substance in a paper on picric acid and the picrates*, 
I mentioned that in a great number of specimens examined, no planes 
except those parallel with or perpendicular to the principal axis had 
been met with. Since then I have obtained in several crystalliza- 
tions specimens exhibiting a brachydiagonal doma; but this appears 
to be rather unusual. 
The optical properties of this salt are very interesting. It exhihits 
a beautiful dichroism. If the crystal be viewed by light transmitted 
in the direction of its principal axis, it appears of a pale straw-colour, 
in any other direction, rich aurora-red in some specimens, in others 
salmon-colour. A doubly refracting achromatized prism gives images 
of these two colours, unless the light be transmitted along the principal 
axis of the crystal of picrate, in which case both are pale straw-colour. - 
* Silliman’s American Journal, Noy. 1858. 
