The crystal was now placed with its optic axis parallel to the direction of the incident ray. There was no action on the Galvanometer. Rotation of the crystal round this axis, did not produce any effect on the Galvanometer. The field continued to be dark.
2° Apatite.—This specimen exhibited decided double refraction.
3° Nemalite.—This is a fibrous variety of Brucite. This specimen exhibited a very strong depolarisation effect. It also exhibited certain interesting peculiarities which will form the subject of a future communication.
Rhombic system.—A large piece of Barytes was found strongly double refracting.
Triclinic system.—Microcline, a greenish blue crystal of the double oblique type, exhibited polarisation effect to a remarkable degree.
Regular system.—A large crystal of Rock-salt was taken. This as was expected did not produce any effect.
Having satisfied myself of the fact that systems of crystals other than regular, produce double refraction and consequent polarisation of electrical ray, I tried the action of electric radiation on crystals ordinarily used in optical experiments.
I got a fairly large piece of black Tourmaline. On interposing this with its plane vertical, there was prompt movement of the spot of light. There was no action on the Galvanometer, when the principal plane coincided with the planes of polarisation of either the Polariser or Analyser.
With ordinary light a piece of Tourmaline of sufficient thickness absorbs the ordinary, but transmits the extraordinary ray. With the piece of Tourmaline used in the last experiment I found both the rays transmitted, but, it seemed to me, with unequal intensities. In other words, one ray suffers greater absorbtion than the other. It seems probable that with greater thickness of crystal one ray would be completely absorbed. I found other crystals behaving more or less in the same way. I reserve for another communication particulars of experiments bearing on this subject.
Lastly I tried an experiment with a crystal of Iceland spar, taken out of a Polarising apparatus. With this I got distinct depolarising action.
The above results, with the exception of the last, were obtained with uncut specimens. Their faces were often rough and irregular. Better results, were they needed, could no doubt be obtained by judicious cutting and polishing the faces.
Summary.—It will thus be seen that crystals which do not belong to the Regular system, polarise the electric ray, just in the same way as they do a ray of ordinary light. Theoretically all crystals, with the
