Fesruary 18, 1897] 
MA TORE 
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lately, and has determined the amounts of both copper and iron 
present in various kinds of oysters by electrolytic methods. 
Tle finds the green Marennes oyster contains about 0°4 mgrme. 
(say ‘006 grains) of copper, which agrees pretty closely with 
the figures given by previous writers. This seems to be the 
normal amount present in all oysters, white or green, and due 
to the hemocyanin of the blood. Dr. Thorpe, however, finds 
that the green Falmouth oysters have, on the average, each 
‘023 grains of copper, which falls to the normal amount ("006) 
on re-laying in another locality, and which is ‘‘ obviously caused 
by the mechanical retention of cupriferous particles” (Thorpe, 
Nature, p. 107). If Dr. Thorpe means by this that copper 
mud is entangled in the water and food passages of the oyster, 
is it not possible that, although the oyster is green, and copper 
is present, the colour may be due—as in most green oysters — 
to another cause? This mere entanglement (more or less 
accidental) of copper-bearing material in the passages of the 
oyster may also be the explanation of the extraordinarily high 
figure reported by Mr. Lowe—a figure (-04 grammes) as large, 
I may remark, as that of the foa/ ash in the case of some of my 
oysters investigated by Dr. Kohn. W. A. HERDMAN. 
Liverpool, February 6. 
Immunity from Snake-Bite. 
In regard to the immunity from the danger of a second bite | 
which a non-lethal dose of snake venom affords an animal, and 
also in regard to the question of antitoxin, I would suggest that | 
the comparatively simple case of the sting of bees might be 
investigated. 
The keeper of an apiary once told me that when he first took 
charge of it, he was laid up for some days by the intense in- 
flammation due to the stings, but that he soon became quite 
indifferent to the venom. I myself saw him stung several 
times during a few minutes while he was emptying one hive 
into another. He had no protection over his hands and face, 
and, except for the sharp prick of the actual sting, he suffered 
no ill-effects. 
May not the stinging liquid, generally assumed to be formic | 
acid, be of the same nature as snake venom? Might not formic 
acid have the same effect ? R. G. LT. Evans. 
SUBJECTIVE COLOUR PHENOMENA. 
ey a recent communication to the Royal Society,! I 
described a series of optical experiments which 
originated in an attempt to account for the colour 
phenomena exhibited by Mr. C. E. Benham’s “ Artificial 
Spectrum Top” (NATURE, vol. li. p. 113). The chief of 
these experiments are of an exceedingly simple character, 
and can easily be repeated without the employment of 
any special apparatus. They demonstrate the formation, 
under certain conditions, of transient bands of colour 
along the boundaries between light and dark surfaces. 
Let a hole, half an inch square, be cut with a sharp 
knife in the middle of a sheet of thick brown paper about 
15inches square. The hole is to be covered with gummed 
white paper taken from the edge of a sheet of postage 
stamps (“stamp paper”); a small translucent window is 
thus formed. Across the middle of the window a common 
pin is to be fixed, like a bar, by means of narrow strips of 
stamp paper at its two ends. Holding the brown paper 
in the left-hand between the eyes and a lamp, the 
observer directs his eyes upon the translucent window ; 
then he conceals it from view by interposing a screen, 
such as a thin book with a dark cover. After a few 
seconds, and without moving the eyes in the meantime, 
he suddenly withdraws the screen ; then, if everything 
is right, and the observer is not unaccustomed to subjec- 
tive visual experiments, the window will, for a moment 
after its exposure, appear to be surrounded by a narrow 
red border, while the pin also will at first appear bright 
red, not turning black until after the lapse of about one- 
tenth of asecond. The effect is seen best when the lamp 
is at a certain distance from the brown paper. This dis- 
1 “On Subjective Colour Phenomena attending sudden Changes of Illum- 
ination.” (Proc. Roy. Soc., December 17, 1896.) 
NO. 1425, VOL. 55] 
tance must be found by trial ; in my own case an eight- 
candle power lamp gives good results when it is about 
12 inches behind the paper. The observer’s eye should 
be 10 or 12 inches away from the translucent window. 
When once the red border has been detected, it 
becomes very conspicuous; the difficulty in the first 
instance being not to see it, but to know that one sees it. 
The phenomenon is, without doubt, constantly met with, 
and habitually ignored, in daily life. Since my first 
observation of it I have many times noticed flashes of red 
upon the black letters of a book, or upon the edges of 
the page: bright metallic or polished objects often show 
a red border when they pass across the field of vision in 
consequence of a movement of the eyes, and it was an 
accidental observation of this kind that suggested an 
experiment like the following :— 
Holding the brown paper between his eyes and the 
lamp, as before, the observer moves it rather quickly 
either up and down, or round and round ina small circle an 
inch or two in diameter. The moving window will, owing 
to persistence, form a straight or circular luminous streak, 
which will appear to be bordered on both sides with 
bright red. No person, however unpractised, to whom I 
have shown this experiment, has failed to see the red 
border at once. As before, the intensity of the illumina- 
tion must be properly regulated ; so also must the speed 
of the movement. With strong illumination the red 
border is very narrow, and is lined with greenish-blue ; or 
the red colour may even be altogether absent. 
The above experiments show that when a luminous 
image (not too bright) is suddenly formed upon the retina, 
it appears at first to be surrounded by a red border. 
The following is a way of showing the same effect by 
reflected instead of by transmitted light. Two or three 
black lines, about as thick and as long as an ordinary 
pin, are drawn upon a small piece of white paper, which 
is placed upon a table and illuminated by strong lamp- 
light (not daylight). A black book is interposed between 
the observer's eyes and the paper, and then very suddenly 
withdrawn ; the lines, when first seen, appear to be red, 
quickly changing to black. So far the observation is a 
rather difficult one, but by a very simple device it is 
possible to obliterate the image of the lines before the 
redness has had time to disappear; the colour then 
becomes easily perceptible. A thin black book is held 
horizontally in the right hand by its left-hand bottom 
corner, the thumb being uppermost ; between the thumb 
and the book is inserted the right-hand bottom corner of 
a sheet of white note-paper; the upper right and left 
corners of the paper and the book respectively are 
separated, so as to form a triangular open space between 
them. The book is held an inch or two above the black- 
lined paper, covering it completely ; then the hand is 
quickly moved from left to right in such a manner that 
the lines are for a moment exposed to view through the 
gap between the book and the note-paper, the movement 
being stopped as soon as the lines are covered by the 
paper. During the brief glimpse that will be had of the 
lines while they are beneath the gap, they will, if the 
illumination is correct, appear to be of a brilliant red hue. 
It must be ascertained by a preliminary trial that neither 
the book nor the note-paper casts a shadow upon the 
black lines when the gap is passing over them. 
By a further simple contrivance the red images may be 
made visible almost continuously for an indefinite time. 
Upon a disc of white cardboard, from 3} to 6 inches in 
diameter, two straight lines are drawn from the centre to 
the circumference, containing an angle of about 45° ; the 
portion enclosed by the lines is cut out nearly up to the 
centre, a rim about + inch wide being left at the circum- 
ference ; the remainder of the disc is divided into two 
equal parts by a straight line from the centre to the 
circumference, opposite the opening, and one of these 
parts is painted black with ink. A pin is passed through 
