Appearances J ram the Contemphitioii of' Coloured Oojrcfs. 337 



has therefore, in this part of tlie inquiry, merely expressed 

 what has been long known, in language physically incorrect, 

 and calculated to convey very erroneous notions on the sub- 

 ject." 



Indeed, the philosophers the author speaks of, might have 

 made the answer he attributes to them (except however Buf- 

 fon, for the theory of the relative insensibility did not appear 

 till a long time after), because they would have supposed tlifit 

 the effect was observed by casting the eye on a white surface. 

 But if they had been asked what should be the effect in a 

 total obscurity, they would doubtless have said that the deci- 

 sion depended on experience. I have therefore not merely 

 expressed \chat has been long know7i. 



We now come to the other objections. 



" Plateau maintains," says the anonymous author, " that 

 after the direct or positive impression of the primitive colour 

 has continued visible for a certain time, and gradually faded 

 away, it is succeeded b}' the negative impression, or acci- 

 dental colour. But what is original in his theory he maintains, 

 that after the accidental colour has faded away in its turn, ii 

 is succeeded by the jyrimitive colour, this alternation going on 

 till the impression wears awa\'. If we look into the volume 

 already quoted *, we shall see that the only novelty in Pla- 

 teau's theory is the lecurrence of the primitive impression, 

 and the continued alternation of the two; but we do not think 

 that this recurrence and alternation are established by suffi- 

 cient evidence; at least, we cannot by any contrivance render 

 it visible. It is certain that the accidental colour disappears 

 and returns, and undergoes other changes ; but these changes, 

 we conceive, are not the effect of the primitive impression, or 

 a continuation of a necessary series of changes, of which the 

 primitive impression is the commencement ; but the result 

 of subsequent actions upon the retina, which M. Plateau has 

 not been careful enough to detect and analyse. It has been 

 })roved, for example, that a pressure upon the retina with the 

 finger changes the accidental colour, and it is asserted by Sir 

 Charles Bell, that if we squint or distort the eye, a vivid im- 

 pression on the retina instantly disappears, as if it were wiped 

 out. When M. Plateau, therefore, saw the accidental colour 

 change into the primitive, was he sure that there was no press- 

 ure made upon the retina by the motion of the eye, or by 

 the involuntary closing even of the eyelids, which is sufficient 

 to produce the observed change? That the changes of colour 



• Tlii-i woik is tlie Treati-c on Optics of Sir David Brevvster, foniiinij 

 a p;irt of tiu; Cahitict ( "vciopjcdia, and in wiiidi tliis philosopher has also 

 j):c-.(MiiP(l a thcor}' ofafcitlental coloiii's. 



Phil. Mag. S. 3. Vol. 1 1-. \o. DO. I\Iai/ 1839. A 



