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Page:Proceedings of the Royal Society of London Vol 60.djvu/467

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exhibited by certain Polarising Substances.
Specimen A. Deflections. Resistance between two opposed faces 2·56 x 1·1 cm. separated by 2·56 cm. Ratio of the conductivities.
In the direction of transmission . .
  ,,   ,, absorption . .
2136 megohms
154   ,,
1 : 13·8

Specimen B. Deflections. Resistance between two opposed faces 2·76 x 1·2 cm. separated by 2·76 cm. Ratio of the conductivities.
In the direction of transmission . .
  ,,   ,, absorption . .
1983 megohms
150   ,,
1 : 13·4

One of the strongest polarising substances I have come across is the crystal epidote. The crystal is very small in size, and I could not get with it complete absorption of one of the two rays. But it exhibits very strong depolarisation effect, even with a thickness as small as 0·7 cm. This is, undoubtedly, due to strong selective absorption in one direction. I cut a square from this crystal 0·7 X 0·7 cm. with a thickness of 0·4 cm. Using an E.M.F. of 14 volts the deflections obtained (proportional to the two conductivities) were 105 and 20 divisions respectively. The conductivities in the two directions are, therefore, in the ratio of 5·2 : 1. With an E.M.F. of 100 volts and a diminished sensibility of the galvanometer, the deflections were 205 and 40, the ratio of the conductivities being as 5·1 : 1.—January 28, 1897.]

It would thus appear that substances like nemalite which polarise by double absorption, also exhibit double conductivity. It is probable that, owing to this difference of conductivity in the two directions, each thin layer unequally absorbs the incident electric vibrations; and that by the cumulative effect of many such layers, the vibrations which are perpendicular to the direction of maximum conductivity are alone transmitted, the emergent beam being thus completely polarised.

[Owing to the great difficulty in obtaining suitable specimens, I have not been able to make a more extended series of determinations. The relation found, in the cases described above, between double absorption and double conductivity is, however, suggestive.