prism in my hand, and turned it to and fro slowly about its axis, so much as to make the several parts of the image, cast on the second board, successively pass through the hole in it, that I might observe to what places on the wall the second prism would refract them. And I saw, by the variation of those places, that the light tending to that end of the image, towards which the refraction of the first prism was made, did in the second prism suffer a refraction considerably greater than the light tending to the other end. And so the true cause of the length of that image was detected to be no other than that light consists of rays differently refrangible, which, without any respect to a difference in their incidence, were, according to their degrees of refrangibility, transmitted towards divers parts of the wall.

Light, therefore, is not similar, or homogeneal, but consists of difform rays, some of which are more refrangible than others: so that of those, which are alike incident on the same medium, some shall be more refracted than others, and that not by any virtue of the glass, or other external cause, but from a predisposition, which every particular ray has to suffer a particular degree of refraction.

As the rays of light differ in degrees of refrangibility, so they also differ in their disposition to exhibit this or that particular colour. Colours are not qualifications of light, derived from refractions, or reflections of natural bodies (as it is generally believed), but original and connate properties, which in divers rays are diverse. Some rays are disposed to exhibit a red colour, and no other; some a yellow, and no other; some a green, and no other; and so of the rest. are there only rays proper and particular to the more eminent colours, but even to all their intermediate gradations.

Nor

There are two sorts of colours: the one original and simple, the other compounded of these. The original or primary colours are red, yellow, green, blue, and a violetpurple, together with orange, indigo, and an indefinite variety of intermediate gradations.

But the most surprising and wonderful composition was that of whiteness. There is no one sort of rays which alone can exhibit this. It is ever compounded, and to its composition are requisite all the aforesaid primary colours, mixed in a due proportion. I have often with admiration beheld, that all the colours of the prism being made to converge, and thereby to be again mixed as they were in the light before it was incident upon the prism, reproduced light, entirely and perfectly white, and not at all sensibly differing from a direct

light of the sun, unless when the glasses I used were not sufficiently clear; for then they would a little incline it to their colour

Why the colours of the rainbow appear in falling drops of rain, is also from hence evident. For, those drops which refract the rays disposed to appear purple, in greatest quantity to the spectator's eye, refract the rays of other sorts so much less, as to make them pass beside it; and such are the drops on the inside of the primary bow, and on the outside of the secondary or exterior one. So those drops, which refract in greatest plenty the rays apt to appear red, towards the spectator's eye, refract those of other sorts so much more, as to make them pass beside it; and such are the drops on the exterior part of the primary, and interior part of the secondary, bow.

I might add more instances of this nature; but I shall conclude with this general one, that the colours of all natural bodies have no other origin than this, that they are variously qualified to reflect one sort of light in greater plenty than another. And this I have experimented in a dark room, by illuminating those bodies with uncompounded light of divers colours. For, by that means, any body may be made to appear of any colour. They have there no appropriate colour, but ever appear of the colour of the light cast upon them, but yet with this difference, that they are most brisk and vivid in the light of their own day-light colour.

Account of the Rotation of a large permanent Spot in the Planet Jupiter, observed by Signor CASSINI.-[1672.]

AMONG the spots of Jupiter, there is none so sensible as one that is situated between the two belts, which in the disk of Jupiter are usually seen extended from east to west; the largest of which is between the centre and the northern limb, and the narrowest is beyond the centre towards the southern limb. This spot is always adhering to the southern belt, its diameter is about the tenth part of that of Jupiter; and at the time that its centre is nearest to that of Jupiter, it is distant from it about the third part of the semidiameter of that planet.

Signor Cassini, after he had made many observations of this spot during the summer of the year 1665, found that the period of its apparent revolution is nine hours and 56 minutes. By the calculation he made in six years, it is found to have made, in respect of the earth, at least 5294 revolutions, each

of nine hours, 55 minutes, 58 seconds, compensating one revolution by another, and at most 5295 revolutions of nine hours, 55 minutes, 51 seconds; forasmuch as he was assured of the preciseness of one mean revolution to one eighth of a minute, which will be verified by future observations.

Some Observations about Shining Flesh, made by Mr.BOYLE.— [1672.]

MR. BOYLE observed a neck of veal to shine in as many as 20 places, though not all alike, as rotten wood or stinking fish do. When all these lucid parts were surveyed at once, they made a very splendid show; so that applying a printed paper to some of the more luminous spots, he could plainly read divers letters of the title. But notwithstanding the vividness of this light, it did not yield the least degree of heat to the touch; and applying to the most shining places a sealed weather-glass, the tinged spirit of wine was not observed to be sensibly affected; and notwithstanding the great number of lucid parts, not the least degree of stench was perceivable to infer any putrefaction.

One of the luminous parts, which proved to be a tender bone, and of the thickness of a half-crown piece, appeared to shine on both sides, though not equally; and the part of the bone whence this had been cut off, was seen to shine, but not near so vividly as the part taken off did before. It yielded no luminous juice, or moist substance, as the tails of glow-worms do: upon compressing a piece of the luminous flesh between two pieces of glass, its light was not extinguished; and putting a luminous piece into a crystalline phial, and pouring on it a little pure spirit of wine, and shaking them together, in about a quarter of an hour or less, the light was vanished. But water could not so easily destroy this light; for putting one of the pieces into a china cup, almost full of cold water, the light did not only appear through that liquor, but above an hour after it was vigorous enough not to be eclipsed by being looked on at no great distance from a burning candle. On conveying one of the larger luminous pieces into a small receiver, the pump was plied in the dark, and on the gradual removal of the air, there was perceived a gradual diminution of the light, though it never quite disappeared, as the light of rotten wood and glow-worms were observed to do; but by the hasty increase of light, that disclosed itself in the veal upon admitting the air into the

exhausted receiver, it appeared that the decrement, though but slowly made, had been considerable.

A luminous piece of it included in a phial, after three days shone as vigorously as ever: the fourth day its light was also conspicuous, so that it could be seen even in the day-time, in a dark corner of the room; but before the ensuing night the light began to decay, and the offensive smell to grow somewhat strong; which seems to argue, that the disposition, by which the veal became luminous, may very well consist, both with its being, and not being, in a state of putrefaction, and, consequently, is not likely to be derived from the one or the other.

Observations on the Nature of Snow.

[1673.]

By Dr. GREW.

IF Aristotle and Descartes, &c. who have written of meteors, and amongst them of snow, have not yet given a full account of it, it will not be needless to enquire further of it. He that will do this, will do it best, not by the pursuit of his fancy in a chair, but with his eyes abroad; where if we use them well fixed, and with caution, and this in a thin, calm, and still, snow, we may by degrees observe, 1st, with M. Descartes and Mr. Hook, that many parts of snow are of a regular figure; for the most part, as it were, so many little rowels or stars of six points; being perfect and transparent ice, as any we see on a pool or vessel of water. On each of these six points are set other collateral points, and those always at the same angles as are the main points themselves. Next, among these irregular figures, though many of them are large and fair, yet from these taking our first item, many others, alike irregular, but much smaller, may likewise be discovered. Again, among these not only regular, but entire parts of snow, looking still more warily, we shall perceive that there are divers others, indeed irregular, yet chiefly the broken points, parcels, and fragments of the regular ones. Lastly, that besides the broken parts, there are some others which seem to have lost their regularity, not so much in being broken, as by various winds, first gently thawed, and then frozen into little irregular clumps again.

From hence the true notion and external nature of snow

seems to appear, viz. that not only some few parts of snow, but originally the whole body of it, or of a snowy cloud, is an infinite mass of icicles regularly figured; that is, a cloud of

vapours being gathered into drops, the said drops forthwith descend; on which descent, meeting with a soft freezing wind, or at least passing through a colder region of air, each drop is immediately frozen into an icicle, shooting itself forth into several points on each hand outward from its centre; but still continuing their descent, and meeting with some sprinkling and intermixed gales of warmer air, or in their continual motion and waftage to and fro touching upon each other, some are a little thawed, blunted, frosted, clumpered, others broken, but the most clung in several parcels together, which we call flakes of snow.

It being known what snow is, we perceive why, though it seems to be soft, yet it is truly hard, because true ice; seeming only to be soft, because on the first touch of the finger on any of its sharp edges or points, they instantly thaw; otherwise they would pierce our fingers like so many lancets. Why again, though snow be true ice, and so a hard and dense body, yet very light, because of the extreme thinness of each icicle in comparison of its breadth. Also how it is white, not because hard, for there are many soft bodies white; but because consisting of parts all of them singly transparent, but being mixed together appear white, as the parts of froth, glass, ice, and other transparent bodies, whether soft or hard.

Thus much for the external nature of snow; let us next a little enquire into its essential nature. Now if we would make a judgment of this, I think we may best do it by considering what the general figure of snow is, and comparing the same with such regular figures as we see in divers other bodies. As for the figure of snow, it is generally one, viz. that which is above described: rarely of different ones, which may be reduced chiefly to two generals, circulars and hexagonals, either simple or compounded together. More rarely, either to be seen of more than six points; but if so, then not of 8 or 10, but 12. Or in single shoots, as so many short slender cylinders, like those of nitre. Or by one of these shoots, as the axle-tree, and touching upon the centre of a pair of pointed icicles, joined together as the two wheels. Or the same hexagonal figure, and of the same usual breadth, but continued in thickness or profundity.