796 PROFESSOR PIAZZI SMYTH ON COLOUR, 
Still more remarkably, an exquisite yellow-green muslin, in eye-colour like 
ereen of young lettuce leaves in a delicately prepared salad,—the most attrac- 
tive thing therefore that muslin, among all green muslins, ever invented to 
induce young ladies into adopting it for a ball dress,—it not only refused to 
turn red at all, but went the opposite way and became of a blue green. I sent 
it therefore to my young friend Dr. PREvost, now prosecuting chemistry in 
Oxford, requesting him to pronounce, if he could by some not too difficult 
chemical analysis, what the colouring material might be ? 
Almost by return of post he answered, “Oh! you need not mention my 
name, for the detection of the chief ingredient was so extremely easy. I merely 
proceeded in the usual elementary chemical manner, then applied Marsn’s test, 
and ‘arseniate of copper’ was abundantly manifested.”* 
SUBJECTIVE CONDITION OF THE COLOUR-BLIND, SO-CALLED. 
From the times of Datton, and through the epochs of HERSCHEL, BREWSTER, 
and TroucutTon, down to the late GeorcE WILsoN, and the happily still exist- 
ing Professors KELLAND and CLERK MAxwELL,—an immense deal of learned 
interest has always been expressed or taken in the peculiar optical condition 
under which some persons are born, and remain throughout life; and which 
has been called for shortness, though with eminent inaccuracy and even mis- 
chievous effect, “ colour-blindness.” 
For some years past, the /aws of such reputed incapacity for seeing colour, 
have been so exactly formulated in various scientific treatises, that there seemed 
little or nothing more remaining to be learnt. And yet I suspect, that the new 
instrument of investigation which the di-chroic spectrum principle puts into 
our hands, will require some of those laws to be re-written; and varieties of 
kind, as well as degree, to be recognised amongst different patients. 
This new necessity however arises very excusably out of the almost impos- 
sible conditions of the problem as hitherto presented. You may, for instance, 
question a colour-blind person to any extent; but if he has never, from his 
birth, seen any difference between two colours, which to you are as opposite as 
any two can be,—how can he describe what he sees in terms of your impres- 
sions? But if you are now enabled by the di-chroic medium to make, or un- 
* Shortly after the above was written I had the opportunity in Lisbon, with a bottle of this 
Judson’s green to my eye, to see agaves, palm-trees, acacias, Indian corn, myrtle bushes, and oleanders, 
not green, but of every description of red from vermilion and cochineal, to deep Indian-red. Yet the 
green-sea water of the Tagus, absolutely refused to alter; though yellow-brown, unpainted oars, dipped 
into it, came up blood-red at every stroke! 
In the Bay of Biscay, where the dull blue waves were running rather high under a cloudy sky, 
and brown sea-weed, like coils of hempen whale line, was floating about,—the bottle showed the waves — 
of an ultra dark, sombre, almost threatening black-green, but the sea-weed of a magnificent coral red, 
almost luminous in the splendour and glory of its redness. 
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