TRANSACTIONS OF THE SECTIONS. 31 



rapidly viewing every point of its surface. Secondly, when the two surfaces have 

 different shades or coloiu-s, the retina of one eye is constantly losing and recovering 

 the vision of one of them. Each optic nei-ve is conveying to the brain the sensa- 

 tions of a different tint or colour. The brain is therefore agitated sometimes with 

 one of these sensations and sometimes with the other, and sometimes with both of 

 them combined, and it is therefore not an imreasonable conclusion that, in the dazzle 

 produced by this struggle of flickeiing sensations, something like lustre may be 

 produced. In studying the subject of lustre there ai-e some facts deserving of atten- 

 tion. In a daguerreotype, for example, of two figm-es in black bronze with a high 

 metallic lustre, it is impossible by looking at either of the pictiu-es to tell the ma- 

 terials of which they are made. No lustre is visible ; but when the two equally 

 shaded pictm-es are combined in the stereoscope, the lustre and true character of 

 the material is instantly seen. Another insti-uctive example is seen in the stereo- 

 scopic representations of a boy blowing a soap-bubble. The lustre of the watery 

 sphere is not visible in either of the two pictures ; but when they are combined, it is 

 distinctly seen. In both these cases, and others of the same kind, tints of similar 

 intensity are combined ; and there is no gi-oimd fora ssimiing that the two surfaces 

 combined appear at different distances, and that the one is seen through the other, 

 as in Professor Dove's theory. 



Observations upon the Production of Colour hy the Prism, the Passive Mental 

 Effect or Instinct in comprehending the Enlargement of the Visual Angle, ami 

 other Optical Phenomena. By J. Alexander Davies. 



The commimication was intended to show that the doctrine of the decomposition 

 of light is not the only possible explanation of colour, but that two causes may, in 

 the way of possibility, be assigned to its production, of which the other is, that the 

 rays receive certain affections or dispositions by their transit through a prism_ or 

 other media. It was not affii-med that the present doctrine, which of course im- 

 plies previous combination or composition, is not the probable one, but only that 

 the idea of its necessary exclusiveuess, as the only one which can philosophically 

 be maintained, is a philosophical error. The difficulty of imagining decomposition 

 in some cases, as, for example, when the solar rays pass through a piece of smoke- 

 blackened glass, was refen'ed to as affording some presumption for supposing that 

 the production of colour by the prism is not occasioned by decomposition, and this 

 especially when it is considered how difficult it is to conjecture how the prism 

 effects the disintegTation of the incident light. The equal difficulty appertaining 

 to the hj-pothesis of disposition was also allowed ; and it was shown that upon 

 either explanation it must be gi'anted that the incident rays pass to the second dyes 

 of the prism, and back again to the first, before they are decomposed, or colours 

 are otherwise produced, and that probably they arise from the backward transit of 

 the rays, which is probably a species of retrogression. The phenomenon, that only 

 the contours and internal lines and points of objects and pictures are coloured when 

 seen through a prism, was accounted for by supposing that the rays proceeding 

 from them are prevented fi'om being recomposed by reason of the disturbance of the 

 sun'ouuding colour, which is not affected when seen through a prism, because the 

 various rays are, by the law of chromatic aberration, united after being decomposed 

 by it. The comprehension of the visual angle, or the determination of the prolon-. 

 gation of the angular space, in eveiy case of reilexion and refi-action, was set down 

 either to passive mental action or instinct, and this on the gi-oimd of there not being 

 any physical bai-rier, and from the fact that single vision alone is sufficient to pro- 

 duce this effect. 



The light proceeding from luminous objects was stated to be accompanied with 

 colour, and not colour per se : and as regards the intensity of colour, it was con- 

 cluded that, as an example, a thin mixture of Indian ink is caused either by the 

 very thin distiibution of black particles, or white or almost white ones, more or less 

 closely compacted ; supposing which to be the case, the mixture is darkened with 

 every' increase in their compactness ; of which explanations the former was con-, 

 sidered to be the correct one. 



The fact that black polished surfaces, however great the approximative perfec- 



