On the Cubical Compressibility of solid Homogeneous Bodies. 447 
particular characteristics; but in some of the leaves, especially 
in their thicker parts, natural fissures were discovered ex- 
tremely narrow, certainly less than 3,55 millim. across, which 
were clearly polarized, especially towards their narrowest ends, the 
plane of polarization being, as before, perpendicular to the direc- 
tion of the slit. These polarized slits are rather rare in gold- 
leaf; and I never found them in the thinner and more transpa- 
rent parts, but only in those whose thickness cannot be much 
less than 3,555 millim. This property seems to indicate that 
the phenomenon is only produced when the layers have a certain 
thickness, below which it is insensible. 
The same conclusion was come to on observing the thin layers 
of silver already mentioned. In fact layers C and D, which 
were excessively thin and transparent, exhibited no sensible 
polarization even in their finest slits ; while the somewhat thicker 
and more opake layer A, whose thickness amounted to gqb5 
millim., exhibited the phenomenon in a remarkable manner, and 
with certain curious peculiarities. Some of the narrowest of the 
lines which penetrated through the entire thickness of the metal 
were polarized, some partially, others almost totally, the plane 
of polarization being still perpendicular to their length. On 
employing solar light, the phenomenon of coloured light appeared 
in conjunction with that of polarization; for on observing the 
polarized lines with a doubly-refracting prism, the two images 
appeared in certain cases of complementary colours. 
I believe I have now communicated the principal facts which 
I have discovered relatively to the polarization, 
1. Of light dispersed from furrowed metal surfaces, 
2. Of the rays regularly reflected by such surfaces, 
3. Of light which has traversed very narrow slits. 
With the permission of the Academy I shall for the present 
content myself with the foregoing statement of facts, without 
entering on the premature and, as yet, uncertain question of the 
causes and connexion of the phenomena above detailed. 
LXVII. On the Cubical Compressibility of certain sold Homo- 
geneous Bodies. By M. G. WertHEIM*, 
1848 I published a memoir on the proportion between 
the elongation and transverse contraction of a homogeneous 
isotropic elastic bar when subjected to longitudinal traction. 
After having called attention to the fact that the value of this 
ratio as determined by Poisson’s analysis, viz. 4, had never been 
confirmed by any conclusive experiments, I shewed in the case 
of certain substances which I was enabled to submit to direct 
* Translated from the Comptes Rendus, vol. li. p. 969, 
