14- Mr. G. H. Fielding's Reply to Sir David Brewster. 



as to form an opening tor the glass rod to pass through it, in 

 order that the rectangle may hang perpendicularly. A similar 

 rectangular closed conductor is supposed to be connected 

 with the poles of another battery. 



If the last conductor be placed above that represented in 

 the figure, but not parallel to it, as in the case of stable equili- 

 brium, the moveable conductor will turn round till that posi- 

 tion be gained. But the moveable conductor being put in 

 motion will pass this position at the moment the ends of the 

 wires, a b, pass above the two divisions in the groove, so that 

 the direction of the voltaic influence is changed, and the 

 moveable conductor forced round another semicircle, when 

 the direction of the influence is again changed; and so on, 

 producing continued rotation. By using a magnet instead of 

 the closed conductor, a more powerful and rapid rotation 

 may be produced. By placing the divisions in the magnetic 

 meridian, the closed conductor may be made to revolve by 

 the action of the earth. 



This experiment, if I mistake not, will afford an interesting 

 illustration of the mutual action of voltaic conductors, and 

 of the striking analogy between a permanent magnet and a 

 closed circuit conducting voltaic electricity. 



VI. On the Membrana versicolor of the Eyes of Animals, in 

 reply to Sir D. Brewster. By G. H. Fielding, Esq., 

 M.E.C.S., Curator of Comparative Anatomy to the Hull 

 Literary and Philosophical Society. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 

 TN your Number for October, I observe a communication 

 -*- from Sir David Brewster, which requires a few remarks 

 from me by way of reply. It is entitled, " Notice respecting 

 certain Changes of Colour in the Choroid Coat of the Eye." 

 The principal object of the paper seems to be to claim for Dr. 

 Drummond of Belfast the merit of being the first to notice 

 the curious fact (which I mentioned in the Number of your 

 Magazine for August last, as a discovery of my own,) of the 

 disappearance by drying, and reproduction by wetting with 

 water, of the brilliant colours of the choroid membrane of 

 certain animals. 



I was certainly not aware of the fact, as I had not seen the 

 article "Oitics," of the Edinburgh Encyclopaedia, nor ever 

 heard of the experiment alluded to. The claim, however, is 

 indisputable, and I with pleasure cede the palm of priority 

 to Dr. Drummond. 



