508 Messrs. Gladstone and Tribe on 



the incongruence between the degree of convergence and the 

 parallax of motion is perceived with great accuracy. 



The easiest way to see them is the following : — Look to a 

 papered wall, the pattern of which is regularly repeated at 

 distances not much greater than the distance between your eyes 

 (between GO and 70 millim.). You know that it is possible 

 to make your eyes converge either to a nearer or to a more 

 distant point than the surface of the paper ; so that in your 

 binocular field of vision two images get corresponding posi- 

 tion, which do not belong to the same part of the paper, but 

 to two different copies of the pattern. You see, then, a ste- 

 reoscopic image of the pattern, either more distant and of 

 greater apparent size, if you diverge your eyes, or nearer and 

 smaller, if you converge. But the appreciation of the appa- 

 rent distance of this pattern is not very precise. If you 

 try to bring a pencil to the apparent place of the nearer pat- 

 tern, you will find that the point of convergence is far nearer 

 than the apparent place of the pattern. 



When you now move your head the pattern moves also. 

 If you have increased the convergence of the eyes, the pattern 

 moves with the eyes, 'as well to the left and the right as up 

 and down and forwards and backwards, If you diverge, it 

 goes in opposite direction to your head. 



LXVII. Note on Thermal Electrolysis. 

 By J. H. Gladstone, F.R.S., and Alfred Tribe *. 



DURING the course of our experiments on metallic re- 

 placements we noticed that some sheet silver, immersed 

 in fused silver chloride, became quickly studded with crystals 

 of metal. A 'replacement of a metal by itself seemed so 

 anomalous, that our first idea was that the silver employed 

 contained certain impurities; but we found that the action 

 took place just as well with the purest silver we could ob- 

 tain, and that it was not restricted to the substances above 

 mentioned. Not only might the iodide of silver be substi- 

 tuted for the chloride with the same result, though not so 

 rapidly effected, but other metals might be employed. Thus, 

 when copper was immersed in fused cuprous chloride, crystals 

 of that metal separated ; and similar exchanges took place 

 when zinc was placed in melted zinc chloride, or iron in ferrous 

 chloride in a molten condition. 



It was then thought that a different physical condition 



* Communicated by tho Physical Society, having been read at the 

 Meeting on April 9. 



