Page:On the Production of a 'Dark Cross' in the Field of Electro-magnetic Radiation.djvu/3

154

field could be displaced, and the quadrants a, b, c, or d (see fig. 1) placed opposite to the receiver. In all these cases, even with small displacements, very strong action was produced on the receiver.

From experiments carried out in the manner described above, the outline of a dark cross projected in space was distinctly made out.

A disc of wood, with concentric rings, would probably show the effect equally well. I shall in a future paper send an account of the action of crystals cut perpendicular to the axis placed in convergent or divergent beams of electric radiation.

Some of the investigations on the rotation of the plane of polarisation will, perhaps, be facilitated by an observation of the rotation of the cross. By a modification of this method I am at present trying to detect the rotation produced in a magnetic field.

Addendum, 16th March, 1898.

The production of a dark cross can also be demonstrated by interposing between the crossed polariser and analyser concentric rings of tin-foil mounted on a thin sheet of mica. But greater interest is attached to the exhibition of the phenomenon by double refracting substances, where the axes of elasticity are disposed in radial directions. From the peculiar stresses present, I surmised that woody stems with concentric rings would exhibit the phenomenon above described. Through the kindness of Dr. Prain, I obtained from the Government Botanical Gardens, Sibpore, stems of Pinus longifolia, Swietenia mahogani, Araucaria Cunninghamii, Mangifera indica, Casuarina equisetifolia, Cupressus torulosa and Dalbergia sissoo. The ring systems present in some of these were very regular,