June 3, 1898.] 



SCIENCE. 



775 



merit in this too busy world, and I hope that 

 Professor Patten, if he happens to have hit upon 

 a fruitful idea, will have an equally early op- 

 portunity to secure a hearing. 



While my theory has had much good luck in 

 the way of a favorable reception, it has hitherto 

 been rather lacking in the honor of being at- 

 tacked, lam, therefore, very glad of the oppor- 

 tunity which is now given me for elucidating 

 some of its features. Professor Titchener dis- 

 poses of my hypothesis in summary fashion by 

 saying, first, that my assumed molecules have 

 a suicidal tendency, and, second, that there is 

 experimental evidence against the theory. The 

 reply in the case of both of these counts is very 

 simple. I take the second one first : 



1. There is no experimental evidence against 

 my theory. There is experimental evidence 

 against the four-color component theory of Bon- 

 ders, but it does not hold against my theory, 

 in which there are not two different kinds of 

 white-sensation. Moreover, the attempt which 

 has been made to show that this same evidence 

 does not hold equally against the four-color 

 theory of Hering can hardly be said to be 

 successful. 



2. It is incorrect to say that I assume, among 

 the properties of my photo-chemical substance, 

 a suicidal tendency ; it should be said at most 

 that it has a semi- suicidal tendency. The 

 photo-chemical substance which I assume is as 

 stable as any other physiological substance in 

 its first estate ; it is only that it becomes un- 

 stable after it has suffered a partial decomposi- 

 tion. As a matter of fact, after we have had 

 a vision of blue for a certain length of time we 

 find that it is followed, even though the eyes 

 be closed, by an after-vision of yellow. This 

 is a marked defect in the optical apparatus 

 with which nature has provided us, and a de- 

 fect from which we do not suffer (to any ap- 

 preciable extent) in the case of the other 

 senses ; the sound-sensation of a given note is 

 not followed by an after-clap of a definite 

 other note. Nature might have done much 

 better for us if she had provided some light- 

 process which was not open to this source of 

 error, but as she has not we must do the 

 best we can to make out the character of the 

 process which she has given us. Whatever 



that process may be, it is plainly something 

 such that, after the external world has sent in 

 to us information regarding a given colored 

 surface, retinal equilibrium has to be restored 

 by a subjective vision of the complementary 

 color, even at the cost, if the eyes be open, of 

 making objects which are really white take on 

 a deceptive appearance of being colored. This 

 fact of nature is mirrored in my theory by 

 supposing that after having undergone a partial 

 decomposition the photo-chemical substance 

 concerned becomes unstable and breaks down 

 completely. This is zweckmdssig, because the 

 retina becomes in this way a tabula rasa, and is 

 thus able to give us correct information regard- 

 ing the color that next impinges upon it. It had 

 not occurred to me that the idea of a chemical 

 compound which, on being partly decomposed, 

 left an unstable residue was so recondite a con- 

 ception as to need to be fortified by authority 

 or by example, and, upon consulting the chem- 

 ists whom I have access to, I find that I am 

 right in this view. But if examples are needed 

 they can easily be given in any quantity. 

 Many unstable phenols, as pyrocatechin and 

 pyrogallic acid, form stable compounds when 

 treated with acid chlorides as benzoyl chloride 

 or acetyl chloride. When these compounds are 

 decomposed, so that benzoic acid or acetic acid, 

 as the case may be, is formed, and the stable 

 acids are removed, the very unstable phenol is 

 left in solution. And it is not even necessary 

 to go so far as to organic chemistry to find in- 

 stances. So elementary a process as the re- 

 moval of an atom of oxygen from sulphuric 

 acid leaves an unstable remainder which grad- 

 ually separates into water and sulphurous an- 

 hydride. 



It cannot be too much insisted upon that the 

 after-image which follows the vision of a colored 

 surface is something peculiar and consequently 

 demands something sui generis in the chemical 

 conception which is to account for it. Miiller, 

 in fact, points out that, if the after-image is to 

 be explained by the play of assimilation and 

 dissimilation, the evident objection ijresents it- 

 self that corresponding after-eflTects ought to 

 occur in other regions of the animal mechanism 

 as well. The only way he has of meeting this 

 objection is to say that any explanation of the 



