508 Proceedings of the Royal Society 
The reason why we never see any one part of the overlapping 
stereoscopic objects simultaneously exhibiting either two different 
colours, or an intermediate colour, is a matter more difficult to ex- 
plain; perhaps the following may be considered sufficient explana- 
tion. If the smallest visible point is a point due to the impression 
produced by a single nerve fibre from one of the eyes, then, as on the 
supposition of a decussation of the fibres in the sensory alternate, 
exceedingly small specks of different colours may at any time appear 
intermixed, from the circumstance of the supposed alternate juxta 
position of the individual fibres from each retina in the sensory, 
so, if this supposition is correct, it is evidently impossible that we 
can ever have the impression of two different colours superimposed 
on the same point and at the same moment of time. 
3. On the Products of the Oxidation of Dimethyl-Thetine, 
and its Derivatives. By Prof. Crum Brown and Dr 
E. A. Letts. 
The difficulty experienced in determining the sulphur contained 
in the compounds of dimethyl-thetine (described in a former com- 
munication) by oxidation to sulphuric acid, induced the authors to 
study the effects of various oxidising agents on these compounds. 
By acting on nitrate of dimethyl-thetine with dilute nitric acid, 
two bodies are produced. The one has acid properties, and forms 
a well-marked soluble salt with baryta. The other has neither 
acid nor basic properties. It crystallises in very beautiful needles 
from a hot solution in alcohol. 
By acting on the base dimethyl-thetine with permanganate of 
potash solution, the same crystalline substance is produced, but the 
presence of the acid body could not be ascertained. The oxidation 
of dimethyl-thetine by permanganate of potash takes place in acid 
or alkaline solution and in the cold. 
Chromic acid has no action whatever on dimethyl-thetine further 
than combining with it to form chromate of dimethyl-thetine — a 
yellow gummy substance which refuses to crystallise. The same 
body may be produced by acting on a solution of hydrobromate of 
dimethyl-thetine with chromate of silver. 
Fuming nitric acid dissolves solid hydrobromate of dimethyl- 
thetine without rise of temperature, but with separation of bromine. 
