332 M. Plateau's Defence of his Theori) of llie Visual 



place to a state of nature directly opposite. The first of these 

 two effects constitutes the duration of the primitive i7)iprcssioti, 

 and the second the apparition of^ the complementary accidental 

 colour. These are the most manifest effects ; but afterwards 

 the impression almost always undergoes oscillations more or 

 less regular: frequently the only effect perceived is the acci- 

 dental colour which alternately appears and disappears; but 

 in certain circumstances, the accidental colour alternates 

 with recurrences of the primitive colour, so that the impres- 

 sion then passes alternately into tioo opposite states, till it com- 

 pletely vanishes. 



In the second case, that is, in the passage -juith regard to 

 space, the state of excitement in the retina extends itself to 

 a small distance round the portion directly excited by the 

 light, and beyond that is manifested a state (f nature directly 

 opposite, from which results the sensation of the comple- 

 mentary colour. The first of these two phaenomena consti- 

 tutes irradiation, and the other the second species of acci- 

 dental colours, or the effects, so ably analysed by M. Che- 

 vreul, of the juxtaposition of colours. These two principal 

 phasnomena are commonly the only ones manifested ; but in 

 favourable circumstances, beyond the space occupied by the 

 complementary colour, the j)rimitive colour is again to be 

 found, though much weakened ; so that oscillations as to time 

 are here substituted, to a certain degree, by oscillations as to 

 space, if I may be permitted so to express myself. 



In order to render the statements of the facts more easy, and 

 to recall the opposition between the primitive and comple- 

 mentary impressions, 1 have named the first vositives, and the 

 second negatives. 



Now the discussion of the phaenomena of the first section 

 has induced me to make them depend upon the general prin- 

 ciple which follows : 



When the retina is submitted to the action of the rays of any 

 colour ^whatever, it resists that action, and tends to recover its 

 normal state, with a gradually increasing force. Then, if the 

 organ be suddenly ivithdrawnfrom the exciting cause, it returns 

 to the normal state by a sort of oscillatory motion, the more in- 

 tense as the action has been father prolonged, a motion in 

 virtue of ixhich the impression passes frst from the positive to 

 the ?iegative state, and the?i contimies generally to oscillate, in 

 a manner more or less regular, by becoming weaker, until it has 

 entirely vanished. 



Hence the successive opposite states of the impression would 

 be analogous to the successive positions of a body removeil 

 irom a stable equilibrium, and which alternately transports 



