338 M. Plateau's T)cfence of his Theory of the T'isual 



in question are not regular, and are produced by some irre- 

 gular influence, may be infen-ed from M. Plateau's own ob- 

 servation, that the alternations of colour do not always take 

 place in the same manner, that they vary 'with the setisibility 

 of the eyes, ajid particularly "with the circumstances under 'which 

 the experiment is made; and he afterwards remarks, that the 

 regular alternation of the primitive and accidental colour is 

 the effect most frequently observed. Now this additional fre- 

 quency of one phaenomenon in a series is no proof of a regular 

 law; and when we consider how the retina is affected by the 

 state of the stomach, by the pressure of the blood-vessels, 

 which may, in some cases, be an intermitting or an alterna- 

 ting one, we must demand a series of distinct experiments 

 made with the same result, on the eyes of different observers 

 accustomed to the examination of this class of phaenomena, 

 and aware of the causes which exercise a disturbing influence, 

 before we can admit the conclusion drawn by M. Plateau." 



The anonymous writer is strangely mistaken, if he supposes 

 that I attach any value to the novelty of my experiments. In 

 support of my theory, I should have considered it more ad- 

 vantageous, had I only to relate facts before ascei'tained. For, 

 with respect to pure phaenomena of sensations, more than in 

 any other circumstance, it may be supposed that the author 

 of a theory is influenced in his judgements, in spite of him- 

 self, by the desire of reconciling the facts with his system. 

 Accordingly, in my detailed memoir, I have endeavoured, as 

 much as possible, to establish my statements on anterior ob- 

 servations ; and, when this resource failed me, I was most 

 careful in requesting other persons to repeat my experiments. 

 For instance, with respect to the recurrence of the primitive 

 impression, a recurrence that the anonymous author was not 

 able to observe, 1 have quoted, in my memoir, the following 

 experiment related by Rozier, the editor of the Journal of 

 Physics* : 



" Suppose," says he, " any apartment, either deprived of 

 the sun's light, or at least in the moment when it can be said 

 that it is neither day or night (the experiment succeeds better 

 in the first case). Suppose in this apartment a candlestick 

 furnished with a lighted wax taper; the light of a candle or 

 lamp producing the same effect. Place this candlestick at 

 your feet, on the floor ; look at this light perpendicularly, so 

 that your eyes remain fixed on it without interruption for se- 

 veral moments. Immediately after, place an extinguisher on 

 this light, lift up your eyes to the wall of the apartment, fix 



• See this Journal, tcin. vi. page 486, year 177«J. 



