On the Poison of the common Toad. 155 
saw only one colour, viz. yellow or orange. -When the mid- 
dle of the red space was absorbed by a blue glass, he saw the 
_black line with what he called the yellow on each side of it. 
We are acquainted with another gentleman who has a similar 
imperfection. 
In all the preceding cases there is one general fact, that red 
light; and colours in which it forms an ingredient, are not di- 
stinguishable by those who possess the peculiarity in question, 
Mr. Dalton thinks it probable that the red light is, in these 
ceases, absorbed by the vitreous humour, which he supposes 
may have a blue colour: but as this is a mere conjecture, 
which is not confirmed by the most minute examination of the 
eye, we cannot hold it as an explanation of the phenomena. 
Dr. Young thinks it much more simple to suppose the absence 
or paralysis of those fibrés of the retina which are calculated to 
perceive red; while’ Dr. Brewster conceives that the eye is, in 
these cases, insensible to the ‘colours at the one end of the 
spectrum, just as the ear of certain persons has been proved, 
by Dr. Wollaston, to be insensible to sounds at one extremity 
of the scale of musical notes, while it is perfectly sensible to 
all other sounds. 
If we suppose, what we think will ultimately be demon- 
strated, that the choroid coat is essential to vision, we may 
ascribe the loss of red light in certain eyes to the retina itself 
having a blue tint. If this should be the case; the light which 
falls upon the choroid coat will be deprived of its red rays, by 
the absorptive power of the blue retina, and consequently the 
impression conveyed back to the retina, by the choroid coat, 
will not contain that of red light—LZdin. Journ. of Science. 
ON THE POISON OF THE COMMON TOAD. BY DR. J. DAVY. 
The following is an abstract of Dr. Davy’s paper on. this 
subject, lately read before the Royal Society, : 
The popular belief in the venomous nature of the toad, Dr. 
Davy states, though of great antiquity, has been rejected as a 
vulgar prejudice by modern naturalists, decidedly so by Cu- 
vier; but like many other long received and prevalent opi- 
nions, it is a true one, and the denial of it by philosophers 
has resulted from superficial examination.» Dr. D. found the 
venomous matter to be contained in follicles, chiefly in the 
- cutis vera, and about the head and shoulders, hut also distri- 
buted generally over the body, and even on the extremities. 
On the application of pressure this fluid exudes, or even spirts 
out to a considerable distance, and may be collected in suffi- 
cient quantity for examination. It is extremely acrid when 
applied to the tongue, resembling the extract of aconite in this 
respect, 
