Page:Collected Physical Papers.djvu/134

114 the disc was next placed vertically above or below the axis. The disc was then moved laterally either to the right or to the left. In this way the field could be displaced, and the quadrants a, b, c, or d (cf. fig. 21) 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.

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. I experimented with transverse sections of 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. I was, however, at first disappointed in failing to obtain the results anticipated. But this failure, I subsequently found, was due to the general opacity of the wood which was freshly cut, and which, though apparently dry, contained large quantities of sap in the interior. I then carefully dried some of the specimens, when the stresses present became quite apparent by numerous cracks starting in radial directions. The results obtained with these dried specimens were quite satisfactory.