88 Mr. G. H. Fielding on a nexv Membrane of the Eye. 



to fade; now dip the portion in cold water, and on taking it out 

 you will find the colours have disappeared; dip it again into 

 the acid, and the colours will reappear as if by the touch of a 

 magic wand ; immerse it again in the water, and they will dis- 

 appear ; and so on, as often as you please. The same effect is 

 produced by a solution of ammonia. With a pigment this 

 could not occur ; and my impression is, that these beautiful 

 colours depend upon the thickness and disposition of the thin 

 laminae, of which, by dissection, I can prove this membrane to 

 be composed. The cause of the disappearance and reproduc- 

 tion of the colours by chemical agency, I conceive to be merely 

 the effects of heat and cold upon these thin plates, causing 

 alternate expansion and contraction." Your observation is as 

 follows : " Are not these changes of colour more probably re- 

 ferrible to the alterations of texture necessarily induced upon 

 so delicate an organized structure by the application of che- 

 mical agents ?" 



Now, Gentlemen, the main object of my argument is to 

 prove that the part in question is membrane, and not pig- 

 ment. With this I think you seem to agree, by your use of the 

 words "organized structure;" but you feel disposed to think 

 that the curious changes in appearance in the experiment 

 above detailed, are owing to some actual change in texture 

 produced by chemical means. 



If, therefore, I can show that these changes may be exhi- 

 bited without the intervention of chemical agency, it will suf- 

 fice both to satisfy your objection, and to strengthen my own 

 position. 



In the first place, I should argue that if by any of the means 

 that I have named, we had actually altered the texture of an 

 organized part, we could not have restored it to its primary 

 state again at pleasure ; and therefore, if the colour had once 

 been discharged, because of this actual alteration effected che- 

 mically in the texture, it would have remained discharged. 

 But to prove my opinion that the singular phaenomena de- 

 tailed in the experiment are the simple result of changes pro- 

 duced in the actual thickness of the membrane by alternate 

 expansion and contraction from alternate heat and cold pro- 

 duced by the acid and the water, and not from any chemical 

 effect on the texture of the membrane, I think the following 

 experiment will be sufficient. 



Carefully detach the choroid membrane of a beast's eye 

 from the sclerotica, remove the humours and the retina, and 

 wash off all the brown pigment with water and a camel's hair 

 brush. Then suspend the choroides in a current of air to dry. 

 Observe carefully the nature of the bright colours of the mem- 



