660 



REPORT — 1892. 



are not given ; but the scheme indicates where they cotne, whether helow or above 

 100. la no case were they higher than 200 or lower than 50. 



A few experiments were made on the external change of volume of a few of the 

 tubes, which were inclosed in a thin-walled brass tube. The brass tube yielded 

 because of its thinness, so that the results were not certain. Rut there was no 

 doubt that with the specimens of iron tried there were large changes of volume. 

 Joule's nugatory result must have been peculiar to the kind of iron he worked 

 with. 



4. On Polarising Oratings. By Dr. H. E. J. G. DU Bois. 



Very minute gratings made of silver wire of ^ millimetre in diameter 

 were found to polarise transmitted light perpendicularly to the wires. The 

 amount of polarisation was found within a certain range to be inversely propor- 

 tional to the apparent width of the clear spaces, and to increase with the wave- 

 length, reaching 13 per cent, in one case for red light. The same effect was found 

 with gratings scratched into thin platinum films. In collaboration with Dr. 

 Rubens, the author then extended these researches to heat radiations beyond the 

 red. It was found that the perpendicular polarisation reached a maximum near 

 X = 100 microcentimetres ; thence diminishing and vanishing near X = 200. For 

 greater wave-lengths the polarisation became parallel to the wires, and went on 

 increasing with X. The analogy of these phenomena with the polarisation of 

 Hertzian radiations appears to be more than a superficial one. The authors by 

 developing these preliminary experiments, by using still finer wires made of differ- 

 ent metals and reaching longer waves, hope to contribute towards bridging over 

 the considerable gap that still separates Hertzian electro-magnetic radiation from 

 those radiations with which we are more intimately acquainted. 



5. An Estimate of the Hate of Propagation of Magnetisation on Iron. 

 By Professor G. T. Fitzgerald, F.B.S. 



6. Eemarks on the Early Rise and Progress of Electrical Illumination. 

 By Dr. Alexander Keiller. 



It was pointed out that in 1836 Mr. J. B. Lindsay, of Dundee, gave lectures 

 on electricity accompanied with an exhibition of a constant electric light . 



Report of the Committee on Electro-optics. — See Reports, p. 157. 



8. On Dispersion in Double Refraction due to Electric Stress. 

 By Dr. John Kerr. 



This paper forms the appendix of the report on Electro-optics. — See p. 157. 



