a white ground, as a red wafer on a piece of white paper, you will see, after some time, a blue border around the wafer; if you then turn your eye from the wafer to the white paper, you will see a round spot of delicate green, inclining to blue, which will continue longer according to the time you have looked at the red object, and according as its splendour and brightness have been greater. On directing your eyes to other objects, this impression will gradually become weaker, and at length disappear.

If, instead of a red wafer, you look at a yellow one; on turning your eye to the white ground you will observe a blue spot.

A green wafer on a white ground, viewed in the same manner, will produce in the eye a spot of a pale purple colour: a blue wafer will produce a spot of a pale red.

In the last place, if a black wafer on a white ground be viewed in the same manner, after looking at it for some time with attention, you will observe a white border form itself around the wafer; and if you then turn your eye to the white ground, you will observe a spot of a brighter white than the ground, and well defined. When you look at a white spot on a black ground, the case will be reversed.

In these experiments, red is opposed to green, and produces it, as green produces red; blue and yellow are also opposed, and produce each other; and the case is the same with black and white, which evidently indicates a constant effect depending on the organization of the eye.

This is what is called the Accidental colours, an object first considered by Dr Jurin, which Buffon afterwards extended, and respecting which he trans

mitted a memoir in 1743 to the Royal Academy of Sciences. This celebrated man gave no explanation of these phenomena, and only observed that, though certain in regard to the correctness of his experiments, the consequences did not appear to be so well established as to admit of his forming an opinion on the production of these colours. There is

reason however to believe that he would have explained the cause, had he not been prevented by other occupations. But this deficiency has been supplied by Dr Godard of Montpellier; for the explanation which he has given of these phenomena, and several others of the same kind, in the Journal de Physique for May and July 1776, seems to be perfectly satisfactory.

PROBLEM LXVII.

To determine how long the Sensation of Light remains in the Eye.

THE following phenomenon, which depends on this duration, is well known. If a fiery stick be moved round in a circular manner, with a motion. sufficiently rapid, you will perceive a circle of fire. It is evident that this appearance arises merely from the vibration impressed on the fibres of the retina not being obliterated, when the image of the fiery end of the stick again passes over the same fibres; and therefore, though it is probable that there is only one point of light on the retina, you every moment receive the same sensation as if the luminous point left a continued trace.

But it has been found by calculating the velocity of a luminous body put in motion, that when it

makes its revolution in more than 8 thirds, the string of fire is interrupted; and hence there is reason to conclude, that the impression made on the fibre continues during that interval of time. But it may be asked, whether this time is the same for every kind of light, whatever be its intensity? We do not think it is; for a brighter light must excite a livelier and more durable impression.

SUPPLEMENT,

Containing a short Account of the Most Curious Microscopical Observations.

PHILOSOPHERS were nó sooner in possession of the microscope, than they began to employ this wonderful instrument in examining the structure of bodies, which, in consequence of their minuteness, had before eluded their observation. There is scarcely an object in nature to which the microscope has not been applied; and several have exhibited such a spectacle as no one could have ever imagined. What indeed could be more unexpected than the animals cr molecule (for philosophers are not yet agreed in regard to their animality) which are seen swimming in vinegar, in the infusions of plants, and in the semen of animals? What can be more curious than the mechanism in the organs of the greater part of insects, and particularly those which in general escape our notice; such as the eyes, trunks, feelers, terebræ or augres, &c? What more worthy of admiration than the composition of the blood, the elements of which we are enabled to perceive by means of the microscope;

:.

the texture of the epidermis, the structure of the lichen, that of mouldiness, &c? We shall here take a view of the principal of these phenomena, and give a short account of the most curious observa tions of this kind.

§ I.

Öf the Animals, or Pretended Animals, in Vinegar and the infusions of plants.

1st. Leave vinegar exposed for some days to the air, and then place a drop of it on the transparent object-plate of the microscope, whether single or compound: if the object-plate be illuminated from below, you will observe in this drop of liquor, animals resembling small eels, which are in continual motion. On account of the circumvolutions which they make with their long, slender bodies, they may be justly compared to small serpents.

But it would be wrong, as many simple people have done, to ascribe the acidity of vinegar to the action of these animalcules, whether real or supposed, on the tongue and the organs of taste; for vinegar deprived of them is equally acid, if not more These eels indeed, or serpents, are never seen but in vinegar which, having been for some time exposed to the air, is beginning to pass from acidity to putrefaction.

so.

2d. If you infuse pepper, slightly bruised, in pure water for some days, and then expose a drop of it to the microscope, you will behold small animals of another kind, almost without number. They are of a moderately oblong, elliptical form, and are seen in continual motion, going backwards and forwards in all directions; turning aside when they

meet each other, or when their passage is stopped by any immoveable mass. Some of them are observed sometimes to lengthen themselves, in order to pass through a narrow space. Certain authors of a lively imagination, it would appear, even pretend to have seen them copulate, and bring forth; but this assertion we are not bound to believe.

If other vegetable bodies be infused in water, you will see animalcules of a different shape. In certain infusions they are of an oval form, with a small bill, and a long tail in others they have a lengthened shape like lizards: in some they exhibit the appearance of certain caterpillars, or worms, armed with long bristles; and some devour, or seem to devour their companions.

When the drop in which they swim about, and which to them is like a capacious bason, becomes diminished by the effect of evaporation, they gradually retire towards the middle, where they accumulate themselves, and at length perish when entirely deprived of moisture. They then appear to be in great distress; writhe their bodies, and endeavour to escape from death, or that state of uneasiness which they experience. In general, they have a strong aversion to saline or acid liquors. If a small quantity of vitriolic acid be put into a drop of infusion which swarms with these insects, they immediately throw themselves on their backs and expire; sometimes losing their skin, which bursts, and suffers to escape a quantity of small globules that may be often seen through their transparent skin. The case is the same if a little urine be thrown into the infusion.

A question here naturally arises: ought these moveable moleculæ to be considered as animals? On this subject opinions are divided. Buffon thinks