1835.] Composition of White Light. 185 



4. " These two accidental colours have reciprocally the 

 the same tints as the two real colours ; they are also com- 

 plementary, the one to the other : that is to say, they 

 have the relation of tints which two real colours ought to 

 have to produce white." Hence, *' since any two real com- 

 plementary colours form, together, white, any two accidental 

 complementary colours produce the opposite of white or 

 black." 



5. "In all the cases, where real colours, by combining, 

 produce white, the accidental colours of the same tints pro- 

 duce the opposite, or black." 



6. Hence, the " accidental impression is of an opposite 

 nature to the direct corresponding impression. 



7. Combining the previ6us results, (1, 2, and 7), it may 

 be concluded that when the retina, after having been excited 

 for some time, by the presence of a coloured object, is sud- 

 denly withdrawn from this excitement, the impression pro- 

 duced by the object continues to subsist, during a period 

 which is generally very short ; after which, the retina ac- 

 quires, spontaneously, a state opposite to its first condition, 

 from which the perception of the accidental colour results. 



8. Anew property is thus admitted in the retina; for, 

 ' " the retina opposes to the action of light a resistance which 



increases with the duration of this action, and from which 

 there appears to us, when we look at an object for a long 

 time, a progressive weakening in the brightnessof the object. 



9. Plateau sums up the substance of his theory as fol- 

 lows : When the retina is submitted to the action of the 

 rays of any colour, it resists their action, and strives to 

 regain its normal state with an increasing force. If it is 

 then suddenly withdrawn from the exciting cause, it returns 

 to the normal state, by an oscillatory movement, propor- 

 tionally great according to the prolongation of the action, 

 and by which the impression passes, at first, from a positive 

 to a negative state, then continues generally to oscillate in 

 a manner more or less regular, while weakening ; some- 

 times terminating by disappearing and re-appearing alter- 

 nately ; at other times passing successively from the nega- 

 tive to the positive state, and vice versa. 



The interval which elapses between the instant when the 

 retina is withdrawn from the action of the coloured object. 



