TRANSACTIONS OF THE SECTIONS. 23 



4th ring purple, darker than the 2nd. 



5th ring pink. 



By means of excentric motions a great variety oi colours were obtained ; amongst 

 others, a pure red and various shades of purple, pink, yellow, and blue. The number 

 of discs tried were very great, each disc having on it a different proportion of black 

 and white. 



The author produced the same results by cutting out spaces in the white card, and 

 causing it to revolve on a black surface. He produced also similar phenomena by 

 causing these figures to revolve when held perpendicularly, and to take the appearance 

 of coloured solids. He also caused these colours to be reflected on a white surface 

 from the revolving disc. These experiments, and the views drawn from them, were 

 used for the purpose of giving a theory of the prism, to be published in detail. It 

 was by such processes that the author was led to believe that he had demonstrated 

 that colour is produced by a mixture of light and shadow at various intervals ; and 

 at last he was satisfied that the experiments were original, and not explicable by the 

 present recognized laws. 



He concluded in these words nearly :— Remarkable as these experiments are, they 

 are not more remarkable than the results they lead to. 



They prove the homogeneity of the ether. 



They prove the undulatory hypothesis, but oppose the uudulatory theory. 



They show the necessity of introducing a negative element into the theory of colour, 

 or that colour is the effect of two coordinate sensations— a positive and a negative. 



They enable us to dispense with the different refrangibilities of the rays of light, 

 taught by Newton. 



They remove the necessity for the supposition of different lengths of waves or of 

 a disposition in matter to produce waves of different lengths. 



They help to explain many of the phenomena of what is called the polarization of 



light. 



They give a new explanation of prismatic refraction, and explain in a plain and 

 simple manner many very interesting natural phenomena. 



Startling, he said, as these conclusions are, to those who are conversant with the 

 subject of light, he thought he was perfectly warranted in drawing them from his 

 experiments. 



On Radiant Heat. By B. Stewart, M.A. 



In addition to the facts communicated at the last Meeting, the author mentioned 

 that he had since examined the nature of the heat emitted by heated rock-salt, and 

 found that it possessed very great wave-length. He had also shown that table-salt, 

 pounded saltpetre, and pounded sulphate of potassa were white for heat ; while 

 pounded sugar, pounded alum, and pounded citric acid were black. The inference 

 is that, could saltpetre or sulphate of potassa be obtained in crystals large enough, 

 they would be diathermanous like rock-salt. 



He had also, in endeavouring to ascertain the law of particle radiation, asked him- 

 self the question, What would be the consequence if the ultimate particles of dif- 

 ferent bodies radiated the same quality of heat at the same temperatures ? and he had 

 calculated that were there a group of bodies possessing this common property, viz. 

 having particles which radiate the same quality of heat at the same temperature, it 

 would follow that if we were to take slices of such bodies of thicknesses such that 

 they all permitted to pass the same proportion of heat of any one kind, then they 

 would also all pass the same proportion of heat of any other kind. There are some 

 indications that rock-crystal and glass crystal form one such group, and that citric 

 acid and tartrate of potash and soda form another. 



On recent Theories and Experiments on Ice at its Melting-point. 

 By Professor J. Thomson, M.A. 

 The object of this paper was to discuss briefly the bearings of some of the leading 

 theories of the plasticity and other properties of ice, at or near its melting-point, on 

 speculations on the same subject advanced by the author*; and especially to offer 

 ' * Proceedings of the Royal Society, May 1857 : also British Association Proceedings, 

 Dublin Meeting, 1857. 



