Dr Blitter on a 'peculiar Case Vision. 1S5 
phate of potash, and carbonate of copper been thrown down by 
carbonate of potash, the carbonate of copper has sometimes con- 
tained a sensible portion of arsenic acid. 
I have observed, that arsenic acid, when dissolved in distilled 
water, does not decompose carbonate of potash. 
London, ) 
November 1, 18^1. j 
Art. XXIV . — Remarlcs on the Insensibility of the Eye to cer- 
tain Colours. By John Butter, M. D., F. L. S., M. W. S. 
&c. &c. ; Resident Physician at Plymouth. In a Letter to 
Dr Brewster. 
My Dear Sir, 
Blnowing how much you have directed your attention to the 
subject of optics, and that every yariation connected with the 
ordinary phenomena of vision is interesting to you, I transmit, 
without farther apology, the particulars of the following case, 
which my friend. Dr Tucker of Ashburton, Devon, has lately 
made known to me in the instance of his own son : About two 
years ago, Mr Robert Tucker, who is now aged 19, and the 
eldest member of a family of four children, discovered that he 
was unable to distinguish several of the primitive colours from 
each other. He was employed in making an artificial fly for 
Ashing, intending to have constructed the body of the fly with 
silk of an orange colour, whereas he used that of a green. "When 
the error was pointed out to him by his younger brother, he 
could not believe it, until it was confirmed by other persons. 
Threads of orange and green silk were then twisted round his 
finger, and he could not perceive any difference in them, but 
thought them to be the same coloured thread twisted several 
times. This circumstance led to a trial of his powers for dis- 
tinguishing other colours, and the following are the results which 
have been ascertained, taken correctly by frequent repetition, and 
confirmed by the trials made in my presence. Many of the 
leading or primitive colours, he neither knows when they are 
shewn, nor remembers after they have been pointed out to him. 
I^^ertain colours are confounded with each other. Orange he 
