224 Mr. J. Z. Laurence on the Sensibility 



i. e. blue, produces the violet tint; whilst the yellow mingling 

 with the complementary of red, i. e. green, produces a light green; 

 and this same lasv holds good in the juxtaposition of any two 

 colours whatever. By the term successive contrast Chevreul 

 designates the familiar phenomena of complementary ocular 

 spectra, of which a most comprehensive history has been given 

 by Darwin in the Philosophical Transactions, vol. lxxvi. p. 33 

 et seq. Du Tour* thought that the two eyes cannot perceive 

 each a separate colour at once. He says that if, e. g., a blue disc 

 be presented to one eye and a yellow one to the other, the result 

 is that the mind perceives alternately the one or the other colour, 

 but not the two at once. But I would submit that these two 

 statements do not include the whole facts of the case. I took 

 two tubes, each 10^ inches long, and applying the end of one to 

 each eye, viewed the sky through them. I found that when the 

 ' contiguous edges of the tubes at their further ends were some 

 inches apart, two distinct white circles of sky were seen ; these 

 circles touched when the edges of the tubes Mere from 2^ to 2£ 

 inches apart, and. when closer, the two circles appeared as one. 

 If now the further end of one tube was covered with a piece of 

 green glass, the end of the other with a piece of red, as long as 

 the ends of the tubes were kept not closer than 2\ to 2^ inches 

 asunder the two coloured discs were perceived perfectly distinct 

 from one another; no alternation of either colour to the exclusion 

 of the other, as in Du Tour's experiment, ensued, so long as ihe 

 tubes were inclined to each other at this or any greater degree 

 of divergence. 



Another very interesting series of phenomena depending on 

 the intrinsic sensibility of the eye to the impressions of colours, 

 are those of coloured shadows. The first exact observations on 

 these were made by Count Rum ford f- He observed that the two 

 shadows of an object placed in front of a white ground, from a 

 white and a coloured light, were of the two colours complemen- 

 tary to the latter. I have investigated this fact a little more 

 closely. The method adopted has been to throw a white and a 

 coloured (red) circle of light from two magic lanterns on a white 

 screen, before which a slender wooden rod was placed. It is 

 easy to satisfy ourselves that the red shadow is produced by the 

 (otherwise colourless) shadow cast from the interception of the 

 white light being simply illuminated by the other red light. The 

 green shadow is the shadow produced by the interception of the 

 red light, illuminated by the white light. These coloured sha- 



* Memoires de Mathematique et de Physique presents a VAcademie Royale 

 des Sciences, vol. iii. p. 514 ; iv. p. 4.W. Paris, 17<>0-fi.'-(. 



t Philosophical Papers by Benjamin, Count of Rumford, London, 1802, 

 vol. i. p,335, 



