■526 EEPOET — 1886. 



mometers is of milk-glass, divided into 100 equal parts, both above and below zero, 

 and let into tlie stand so as to constitute a translucent window whicli can be illu- 

 minated from behind. 



By means of this instrument many thermal problems can be demonstrated before 

 .a large audience. 



FBIBAT, SEPTEMBER 3. 



The following Papers and Report were read : — 



1. On the Physical and Physiological Theories of Colour -vision. 

 By Lord Ratleigh, D.G.L., LL.D., Sec.B.S. 



'2. The Modern Development of Thomas Yomig's Theory of Colour-vision. 

 By Dr. Arthur Konig. — See Reports, p. 431. 



3. On the Physical and Physiological Theories of Colour-vision. 

 By Professor Michael Foster, M.D., Sec.B.S. 



4. On Hering^s and Young's Theories of Colour-vision. By John Tennant. 



The author briefly pointed out that, for the purposes of experiments in colour- 

 mixture, Hering's theory, like Young's, has only three independent variables, and 

 leads to the same general results. 



The author then considered the subject of simultaneous contrast, and showed 

 that the only possibility of a deception of the judgment lies in the fallibiUty of the 

 memory. Hence if two patches of the same grey (say) be seen simultaneously 

 they ought, according to the psychological explanation, to appear the same what- 

 ever their surroundings. This he showed, by an experiment with discs, is not the 

 case. Hence simultaneous contrast is a real and not an illusory eiFect, and demands 

 a physiological explanation. 



The author, however, pointed out that there may be illusions of judgment in 

 these cases owing to our habit of attributing permanence to local colour, and that 

 this may account for some of the experiments which have been adduced in favour 

 of the psychological explanation. 



The author then considered some of the difficulties of Hering'a theory, 

 noticing — ■ 



1. That pure red and green are not complementary, as they should be. 



2. A difficulty raised by Rood as to the relative intensity of a compound grey 

 and its components. 



Lastly, the author considered the subject of changes of hue under varying 

 intensity of light. He showed that if we assume that the functional processes of 

 assimilation and dissimilation supposed by Hering draw their materials from, and 

 contribute them to, a common source in the blood, it is natural to suppose that 

 they are not wholly independent. 



He supposed yellow and green to correspond to the dissimilating blue, and red 

 to the assimilating processes in their respective substances. He further supposed 

 the red-green substance to be the most sensitive to drafts upon the common stock. 

 These suppositions are found not only to explain the variations of hue with varying 

 intensity of light, but also that tendency of colours when mixed with white light 

 towards reddish violet, which has received no explanation from current theories. 



The author, without accepting Hering's theory as established truth, thought it 

 the best working hypothesis in the immediate future. 



