340 Prof. Johnston o?i the 



experiment I have seen as many as nine oscillations of the 

 impression produced, that is to say, five transitions from the 

 positive to tiie negative, and four from the negative to the 

 positive. With regard to disturbing causes which might, 

 according to the anonymous author, have occasioned these 

 modifications in tlie impression, I must, if such had pre- 

 sented themselves, have been a very unskilful observer, not to 

 have noticed their influence, nor have secured myself against 

 them. I must moreover say that with respect to myself the 

 accidental colours are not so fugitive, and that they by no 

 means disappear from my eyes by distorting them, squinting, 

 or closing the eyelids. 



The anonymous author is again mistaken, in attributing to 

 me this assertion, that the regular alternation of the priviitive 

 and accidental colour is the effect moU frequently observed. I 

 have, on the contrary, said (page 392 in the article of the 

 Annales), that the effect most frequently observed was that 

 of the disappearances and reappearances of the negative or 

 accidental impression alone. And indeed, among the philo- 

 sophers who have made researches on accidental colours, many 

 have taken notice of this fact, as, for instance, may be seen in 

 the memoirs of ScherfFer and Darwin. 



The reader may see from what precedes, that the facts 

 I have advanced are not deficient in authority, as the anony- 

 mous author seems to allege; he has there limited his ob- 

 jections ; let us, therefore, proceed to those which have been 

 raised by other persons. 



[To be continued.] 



LI 1 1. On the Constitution of the Resins. By James F. W. 

 Johnston, Esq. F.R.S., Professor of Chemistry arid Mine- 

 ralogy in the University of Durham. 



To Richard Phillips, TLsq. 



My dear Phillips. Durham, March 20, 1839. 



'T'HE more I study the constitution of the resins the more 



interesting does it become ; and I begin now to consider 

 them as j)resenting one of the most beautiful natural families 

 to be met with in the entire range of organic chemistry. In 

 my last letter I gave you an outline of some of the results at 

 which I had then arrived. You will be interested probably 

 in learning the further development I have already been able 

 to give them. This will best appear by presenting to you an 

 outline of the classification of the resins, so far as they have 

 yet been analysed. 



General Irrational Formula = C^q Hj^+j. O z/ 



