Mr. G. H. Fielding ofi a new Mcnnbrane of the Eye. 115 



feel quite confident on this head. — 6thly, It is possible by che- 

 mical agents (which, according to Bichat, have not the slightest 

 effect on the pigmentum of the eye) to destroy and restore 

 these colours at pleasure. Take a section of a beast's eye in 

 which the colours are vivid, and dip it into any dilute acid 

 (nitric, muriatic, or sulphuric), you will perceive the colours 

 immediately begin to fade ; now dip the portion in cold water, 

 and on taking it out you will find the colours have disappear- 

 ed ; 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 disappear; and so on as often as you 

 please. The same effect is produced by a solution of ammo- 

 nia. With a pigment this could not occur ; and my impres- 

 sion is, that these beautiful colours depend upon the thickness 

 and disposition of the thin laminee of which by dissection I 

 can prove this membrane to be composed. The cause of the 

 disappearance and reproduction 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 con- 

 traction *. — 7thly, The true pigment of the eye will be found 

 in the same eyes which possess this brilliant substance ; but it 

 will be found behind this part, and most plentiful on the pos- 

 terior surface of the choroid in connection with the sclerotic 

 membrane. Thus, in the ox, this substance presents a fine 

 blue tint intermixed with green and yellow, and behind it we 

 have the true pigment, of a rich brown; in the sheep it is very 

 similar; in the deer the bright part is pale blueish-vvhite, the 

 pigment a very light brown ; in the cat and fox it is a fine 

 golden yellow, and the true pigment a rich black. — Sthly, 

 This membrane (as I must term it) is spread over the whole 

 internal surface of the choroides, next to the Tunica Jacobi or 

 external coat of the retina. It varies in thickness, and conse- 

 quently in the number of its laminae. It is thickest where the 

 lightest and most brilliant colours are seen, ^nd thinnest in 

 the circumference where it appears of an intense blue colour, 

 and where, no doubt, the colour of the Membrana Ruyschiana 

 underneath affects it. It is very remarkable that neither the 

 extent of the bright surface presenting these varying colours, 

 nor the tint of the colours themselves, are uniform in animals 

 of the same species. — Sthly, Minute injection of the choroid 

 does not affect this membrane. — lOthly, 1 have performed all 

 these experiments on the eyes of the sheep and the ox. As 



* Are not these changes of colour more probably reforrible to the alte- 

 rations of texture necessarily induced upon so delicate an organized struc- 

 ture b> the ap[)lication of chemical agents? — Edit. 



