Defence offiis Theory of Accidental Colours* 439 



that even the oscillations, when they do occur, are generally 

 incomplete or irregular, is a complete proof that the theory is 

 not supported by experiment; nay, results directly opposite 

 to the theory are acknowledged to be more frequent than re- 

 sults agreeing with it, and these last are admitted to be gene- 

 rally incomplete or irregular. 



Now, Prof. Plateau admits, that the most frequent result, 

 namely, the disappearance and reappearance of the accidental 

 colour, was observed by many authors, and particularly by 

 Scherffer and Darwin. This is perfectly true, and indeed I 

 cannot conceive how any person could have studied the sub- 

 ject experimentally without observing it. With regard to the 

 rarer phaenomenon which Prof. Plateau and Prof. Quetelet 

 have each seen, and which represents the theory, I have fre- 

 quently observed the one colour succeed the other, but never 

 the whole phaenomenon ; and since I have read Prof. Plateau's 

 writings, I have endeavoured in vain to see it. 



Here then we have two classes of facts ; one of which, viz. 

 the rarer class, leads to one theory, and the other, viz. the more 

 common class, leads to another theory; and Prof. Plateau 

 adopts the theory least supported by facts, solely, we presume, 

 because it appears more beautiful, and more consistent with 

 analogy. 



Regarding the subject as not ripe for generalization, I have 

 ventured to explain the phaenomena of the disappearances and 

 reappearances of the accidental colour, and the occasional 

 succession of the negative and positive impressions, by the 

 operation of certain disturbing causes, which must undoubtedly 

 exercise an influence over all such phaenomena. Prof. Pla- 

 teau answers these arguments by stating, that if such causes 

 had presented themselves, " he must have been a very un- 

 skilful observer not to have noticed their influence, nor have 

 secured himself against them." But observers not very un- 

 skilful have actually found that the accidental colour may be 

 obliterated temporarily or permanently by involuntarily wink- 

 ing, or by closing the eyes with different degrees of pressure, 

 or by distending the eyes, or by knocking the head ; and we 

 believe that the results of all such experiments are affected 

 even by the position of the head, the indirect action of light, 

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

 on the eye-balls. 



Before concluding these observations, I must beg leave to 

 correct a mistake, no doubt accidentally made by Prof. Pla- 

 teau in comparing my theory of accidental colours with his 

 own. " In a certain point of view, as may be seen, Sir David 

 Brewster's theory and mine approach each other ; and indeed, 



