154 "Dark Cross" in the Field of Electromagnetic Radiation. 



FIG. 2. The holder for the disc. D, the paper disc. V, L are the vertical and 



horizontal slides. 



field could be displaced, and the quadrants a, Z>, 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 polar- 

 isation 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 in- 

 terposing 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 phenome- 

 non above described. Through the kindness of Dr. Prain, I obtained 

 from the Government Botanical Gardens, Sibpore, stems of Pinus 

 longifolia, Swietenia niahogani^ Araucaria Cunninghamii, Mangifera 

 indica, Casuarina equisetifolia, Cupressus torulosa and Dalbergia 

 sissoo. The ring systems present in some of these were very regular, 



