438 Sir D. Brewster's Observations oti Prof Plateau's. 



theory to have a better foundation than it has, I must call 

 the reader's attention to the fact, that my mistake was most un- 

 favourable to my own argument ,- and that had I not made the 

 oversight, I should have l)een able to overthrow Prof. Plateau's 

 theory by means of his experiments as well as by my own. 



This theory, as given in § 5, of Prof Plateau's Essai d'une 

 Theorie Ge7ierale, &c. &c., Brussels, 1834, is expressed in the 

 following manner: " The retina returns to its primitive state 

 by a series of decreasing oscillations like a sprijig or a pendu- 

 lu?n; the primitive impression being rapidly effaced in order 

 to be followed by an opposite effect, the nature of which will 

 be determined by experiment ; the primitive impression will 

 then show itself again, but more feeljly, and so on in succes- 

 sion, till the eye cannot distinguish the impressions any 

 longer." The theory therefore is that the primitive and 

 ACCIDENTAL colours follow eack other in regular succession till 

 they disappear altogether. 



After an elaborate and able examination of the various phae- 

 nomena of accidental colours. Prof. Plateau gives the following 

 general result, which, though mixed with much hypothetical 

 language, expresses clearly enough the experimental results 

 at which he has arrived : — 



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

 colour, it resists this action, and tends to resume its ordinary 

 condition with a force more or less intense. If it is then sud- 

 denly withdrawn from the exciting cause, it returns to its 

 ordinary condition by an oscillatory movement, the intensity of 

 which is proportional to the duration of the previous action; 

 a movement, in virtue of whichlhe impression passes, at first, 

 from the positive to the negative state, then continues generally 

 to oscillate iti a manner more or less regidar^ while it becomes 

 weaker and weaker. Sometimes it only disappears and re- 

 appears alternately^ and sometimes it passes successively from 

 the NEGATIVE to the positive state, and vice versa." '■'^ 



Prof. Plateau goes on to state that the " agreement between 

 the results of experiment and the second, hypothesis (the hy- 

 pothesis already given) is remarkable. If the oscillations of 

 the impression which is effaced are generally incomplete or 

 irregular, we must attribute it to causes which the actual state 

 of this part of the science of vision does not yet permit us to 

 appreciate." 



Without taking advantage of the circumstances that Prof. 

 Plateau does not here acknowledge, as he does in the Annates 

 de Chimic-fy that the appearance a7id disappearance of the acci- 

 dental colour is the fact most frequently observed, the admission, 



• Essai, &c. p. 64. f August, 1833, p. 39 



