28 
EXTRAORDINARY FACTS 
relating to the 
Vision of Colours: 
WITH OBSERVATIONS. 
BY MR. JOHN DALTON. 
READ OCT, 31ST, 1704. 
I has been observed, that our ideas of colours, 
sounds, tastes, &c. excited by the same object may 
be very different in themselves, without our being 
aware of it; and that we may nevertheless converse 
intelligibly concerning such objects, as if we were 
certain the impressions made by them on our minds 
were exactly similar. All, indeed, that is required 
for this purpose, is, that the same object should 
uniformly make the same impression on each mind ; 
and that objects which appear different to one should 
be equally so to others. It will, however, scarcely 
be supposed, that any two objects, which are every 
day before us, should appear hardly distinguishable 
to one person, and very different to another, with- 
out the circumstance immediately suggesting a diffe- 
rence in their faculties of vision; yet such is the fact, 
not only with regard to myself, but to many others 
also, as will appear in the following account. 
I was always of opinion, though I might not of- 
