Natural-Philosophical Collections. 469 
that is to say, without being surrounded by other coloured objects. Natural phi- 
losophers give the name of accidental colours to these appearances. ‘This pheno 
menon engaged the attention of Buffon, Scheffer, Darwin, Rumford, M. Prieur, 
Laplace, and others ; but in the circumstances in which these authors examined 
the fact, they could not elicit its whole effects, at least in the case in which the 
eye sees at once two colours in juxtaposition. IM. Chevreul, who, as director of 
the dyes of the royal manufactures, is obliged to look frequently at different co. 
lours, and to view them simultaneously for the purpose of comparing them toge- 
ther, has been induced to examine the subject of accidental colours. He first dis- 
covered the general fact, that two differently-coloured objects placed close to each 
other, always undergo, through the effect of their juxtaposition, a modification in 
their colour. If the one is of a paler colour than the other, the former becomes 
lighter, while the other deepens. MM. Chevreul has determined by experiment 
the reciprocal modifications which the seven primitive colours, as they are called, 
together with black and white, undergo in these circumstances. He has endea- 
voured. to find out the law of these modifications, and has obtained the following 
remarkable result: When two colours A and B are seen at the same time, to 
the colour of A is added the supplementary colour of B, and to the colour of B 
is added the complementary colour of A. 
The colours are therefore seen the most different possible ; and moreover as 
white appears more bright, or a pale colour appears less deep, when they are seen 
simultaneously with a deep colour which itself acquires more intensity, there re- 
sults from this that the contrast takes place with respect to the colour, and with 
kespect to what by the tapestry-manufacturers is named the depth of the tint. 
M. Chevreul remarks that, in the explanation which several natural philoso- 
phers have given of the accidental colours, two very different cases have not been 
sufficiently distinguished. The first is that in which the eye, after looking long, 
for example, on a small square of red paper placed on a white ground, ceases to 
look upon it, and is suddenly directed to some other part of the white ground, 
there is then perceived a small green square, that is, a square of the complemen- 
tary colour of red. We can very easily conceive with P. Scheffer how, in this 
case, the part of the retina on which is painted the image of the red-square, be- 
ing fatigued by this sensation, it happens that the eye ceasing to look at it, and 
always seeing white, the part of the retina, which is fatigued by the red, must 
receive a stronger impression from the complementary rays of the red than from 
these rays themselves, so that the eye must see a green spot. But in the case in 
which M. Chevreul has examined the accidental colours, there are two differently 
coloured and contiguous equal zones which are seen simultaneously; and the 
complementary colour of one of the colours acts, not wpon the part of the retina 
which sees that colour, but upon the part which sees the other colour. The 
learned author intends to recur to the explanation of these phenomena, having 
only accidentally touched upon them in the Memoir which he has read to the 
Academy. 
A part of M. Chevrenl’s Memoir is devoted to the applications of his inquiries 
to the manufacture of tapestry, and, in general, to the optical effects of several 
colours. He explains the differences which the disposition of the same colours, 
and the manner in which they are blended, produce upon the organ of vision. 
Lastly, he gives very simple means of judging of the brightness of the colours of 
a painting, or piece of tapestry, in cases in which the phenomenon in question 
might lead into error, were one desirous of judging of these colours in an absolute 
manner, by looking upon them simultaneously with those in their vicinity. 
New Observations on the Disengagement of Carbonic Acid Gas in Auvergne ; 
by M. J. Fournet.—M. Fournet has made observations in the mines of 
Pontgiband, of which he is director, and in the vicinity, upon the disengagement 
of carbonic gas which fills the crevices, the druses, and, so to speak, the pores of 
the vein. In disengaging itself it emits a whistling sound, and frequently a 
rumbling noise. The mass of the ground and mountain is so saturated with it 
