Appearances from the Contemplation of Coloured Objects. 331 



accide7ital colours by succession, the irradiation, and the. acci- 

 dental colours by simidtaneousness., or the effects of the juxtapo- 

 sition of colours. Above all, 1 should have thought that if 

 my inquiries had given rise to any refutation, it would have 

 been directed against my general principle, against the law 

 of continuity on which I have made these various phsenomena 

 depend : however such has not been the case. It is true, se- 

 veral objections have appeared ; but they merely relate to 

 some particular facts, or at most to that class of phsenomena 

 which constitute the accidental colours of the first-named 

 species. 



Be that as it may, I shall successively examine these dif- 

 ferent objections; but in order to make them more easily ap- 

 preciated, it will be necessary first to restate summarily the 

 principles of my theory. 



I have divided the appearances in question in two large 

 sections : the first comprehends those which succeed the con- 

 templation of objects ; that is to say, the duration of impres- 

 sions, and the accidental colours by succession, or which show 

 themselves after the disappearing of the objects which pro- 

 duce them. In the second are contained the appearances 

 which accompany the contemplation of objects : that is to say, 

 the irradiation, and the accidental colours by simidtaneousness, 

 or which manifest themselves in presence of the objects look- 

 ed at. 



Now, if we consider that the phaenomena of the first section 

 are produced from the moment 'iSohen the object ceases to act 

 upon the retina, to the one in which the organ recovers its nor- 

 mal state, and that, on the other hand, those of the second 

 section surround, on the retina, the space directly excited 

 by the \\g\\\.,from the contour of that space to those parts of 

 the organ -which remaiyi in their normal state, we may say that 

 the first constitute, in the organ, the passage from the state 

 of excitation to the normal state with regard to time, and that 

 the others constitute the passage from the state of excitation 

 to the normal state with regard to space. 



Upon examining the laws which regulate each of these 

 passages, I have remarked a very striking analogy between 

 them ; the one being, as it were, but the translation of the 

 other, by substituting space for time. The facts already known, 

 together with my own observations, have induced me to ac- 

 knowledge that these laws are as follows. 



In the first case, that is, in the passage with regard to time, 

 the retina being suddenly left to itself after an excitement 

 sufliciently proloiigetl, retains for some length of time this 

 same stale of excilcment, which giudually vanishes, to give 



