336 ^I. Plateau's Defence of Ids Theory of the Viswil 



of sensibility of the retina in regard to certain rays, that I 

 have thus made the experiment. 



But the anonymous author alleges that an accidental co- 

 lour cannot be added to, or combined with another. To 

 which I answer by referring him to paragraph 24 of the dis- 

 sertation of Father Scherff'er*, one among the philosophers 

 who have made the greatest researches on the subject of ac- 

 cidental colours, the one precisely who has furnished us with 

 the theory which attributes the phaenomena to a partial dimi- 

 nution in the sensibility of the organ. In this paragraph, 

 Scherffer relates direct experiments whereby he convinced 

 himself that the accidental colours combine perfectly well to- 

 gether. If the anonymous author maintains that these ex- 

 periments can be explained by the diminished sensibility, 

 seeing that Scherffer observed the results of the combination 

 by casting his eye on a white wall, I shall answer that the 

 effects are the same in a complete obscurity, as may be easily 

 ascertained, by substituting for the red and green rectangle 

 of my experiment an orange and green one. Then, if the 

 same process is used, three coloured images will be seen, 

 whose intermediate one will be violet. This image is the re- 

 sult olthe combination of the blue and red accidental colours. 



Consequently, accidental colours are really susceptible of 

 combinations, and the resulting tints are the same as in respect 

 to real colours, except the case in which the two composing 

 accidental ones are complementary to each other. In this 

 latter circumstance, since the result is blackness, it appears 

 to me that I have not expressed the fact erroneously, by saying 

 that two complementary accidental colours produce together 

 black, which signifies, if you will, that they mutually destroy 

 each other. 



It is difficult for me to conceive how the anonymous author 

 has not paid attention to this circumstance formally expressed 

 in my article, that it is in a complete obscurity I have observed 

 the phaenomena. For this is a chief point in my theory of ac- 

 cidental colours, and totally excludes all the explanations 

 which may be founded upon a diminution of sensibility to 

 certain rays. The author thus continues: — 



" If Buffon, or Dr. Darwin, or Count Rumford had been 

 asked what would be the effect of exciting the retina in quick 

 succession with all the simple colours of the spectrum, or 

 with two compound colours which compose white light, they 

 would all have immediately answered, blackness. M. Plateau 



• Journal of Physics, by Rozier, tcni. xxvi. year 1/85, p 583. 



