it is cut out by iron tools, in the size of a cubic foot, or more; and that out of its corners there is sometimes found grown a harder matter capable of cutting glass, of a figure different from that of the whole mass, and approaching to that of diamonds.

The whole body is rather clear than bright, of the colour of limpid water; but that colour, when it has been immersed in water and dried again, becomes dull. Hence it is, that in its native place the upper surface is darkish; because of the rains and snows fallen upon it. Sometimes there appear also some reflections of colours, as in the rainbow. The angles are not pointed alike, all the flat sides being obliquely inclined to one another. The opposite plains are parallel.

In this crystalline prism, two of the plain angles are always acute, and the two other obtuse; and never any of them is equal to the collateral angles of the inclinations.

The objects seen through it appear sometimes, and in certain positions of the prism, double: where it is to be noted, that the distance between the two images is greater or less, according to the different size of the prism, so that in thinner pieces this difference of the double image almost vanishes.

The object appearing double, both images appear with a fainter colour, and sometimes one part of the same species is obscurer than the other.

To an attentive eye, one of these images will appear higher than the other.

In a certain position the image of an object, seen through this body, appears but single, as through any other transparent body.

We have also found a position wherein the object appears sixfold.

If any of the obtuse angles of this prism be divided into two equal parts by a line, and the visual rays pass from the eye to the object through that line, or its parallel, both images will meet in that line, or in another parallel to it.

Whereas objects, seen through diaphanous bodies, are wont to remain constantly in the same place, in what manner soever the transparent body be moved, nor the image on the surface move except the object be moved; we have observed here, that one of the images is movable, the other remaining fixed; although there be a way also to make the fixed image movable, and the movable fixed in the same crystal; and another, to make both movable.

The movable image does not move at random, but always

about the fixed, which while it turns about, it never describes a perfect circle, but in one case.

Of the Abundance of Wood found under Ground in Lincolnshire.[1671.]

THAT fenny tract, called the Isle of Axholme, lying part in Lincolnshire and part in Yorkshire, and extending a considerable way, has anciently been a woody country, as appears by the abundance of oak, fir, and other trees, frequently found in the moor, whereof some oaks are five yards in compass and sixteen yards long; others smaller and longer, with good quantities of acorns near them, lying somewhat above three feet in depth, and near their roots, which do still stand as they grew, viz. in firm earth below the moor. The firs lie a foot or 18 inches deeper, more in number than the oak, and many of them 30 yards long; one of them being, not many years since, taken up of 36 yards long besides the top, lying also near the root, which stood likewise as it grew, having been burnt and not cut down as the oak had been also. The number of these trees is reported by Mr. Dugdale, in his book on draining the Fens in England, to be so great, that the inhabitants have, for divers years last past, taken many cart loads in a year.

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Of the original overflowing of this woody level no account is given. Even Mr. Dugdale only says, that the depth of the moor evinces that it has been so for divers hundreds of years, since that could not grow to the thickness it is of in a few ages. The cause thereof he concludes to have been the muddiness of the constant tides, which, flowing up the Humber into the Trent, left in time so much filth, as to obstruct the currents of the Idle, Done, and other rivers, which thence flowed back, and overwhelmed that flat country.

Of the Stone Quarry near Maestricht.

[1671.]

THERE is an excellent quarry, within cannon-shot of Maestricht, on the very brink of the river Maese, lying in a hill, where there are about 25 fathoms of rock and earth over head; the length of the hill being of some miles, extending along the river towards Liege; and near Maestricht having in breadth some half or three quarters of a mile, but more farther off. This quarry has one entry towards the river, where carts can pass with great ease, and unload the stones on the brink of the river, the quarry within lying

parallel to the horizon or level, and elevated but very little above the river.

This quarry, which has almost undermined the whole hill, affords one of the most surprising prospects, when lighted with many torches, that one can imagine. For there are thousands of square pillars in large level walks, and those almost every where above 20, and in some places many more feet high, and all wrought with so much neatness and regularity, that one would think it had been made rather with curious workmanship for an under-ground palace, than that those pillars and galleries were made by quarriers, that did it only for getting stone to build above ground.

This quarry serves the people, that live thereabout, for a kind of impregnable retreat, when armies march that way. For being acquainted with all the ways in it, they carry into it whatsoever they would have safe, as well their horses and cattle, as their movable furniture, till the danger be over; there being so much room, that 40,000 people may shelter themselves in it.

Mathematical Principles of Light, Colours, the Rainbow, &c. By F. M. GRIMALDI of Bologna. — [1672.]

FATHER GRIMALDI first noticed the spots in the sun, and gave to those in the moon names that are still in use, denominating them after the most eminent astronomers and philosophers. He made numerous experiments in optics, and some discoveries, which were afterwards confirmed and extended by Newton. Grimaldi discovered the circumstance of the lengthening of the solar image, by a ray of light let in through a small hole, and refracted through a glass prism. He taught also that the rays are of different colours, and that opaque objects have no colour but what they receive from the rays of light. He discovered that property of the rays by which, when they pass near the edge of certain objects, though without touching, they are inflected or bent from their direct course, an effect which he termed the diffraction of light, and which Newton afterwards called inflection.

Further, he discourses of colours, and considers how light is changed into colour, sometimes by reflection alone, sometimes by refraction alone, sometimes without either and without the change of the medium, viz. by diffraction. He explains, also, how light, by the sole intrinsic modification of itself, passes sometimes into a colour that is commonly called apparent; where he explains, that the reason why light passes into an apparent colour, is not some determinate

angle at which the rays amongst themselves are inclined, but that that colour is produced by the intention and density of light.

He teaches, also, that to the vision of things permanently coloured, there are not required any intentional species transmitted from them, and contradistinct from light; but that the light which is diffused or at least reflected from things coloured is sufficient; yet with such a modification as is to be found in light apparently coloured, on which occasion many particulars are delivered concerning reflex vision, with an explication of that quære, how the place of the thing seen is perceived, &c.

To all which is added, that the modification of light, by which it is both permanently, and (so to speak) apparently coloured, or made sensible under the representation of colour, may not improbably be said to be a determinate and most finely furrowed undulation of the same, and a kind of tremulous diffusion, with a certain very subtle floating, whereby it does, in a peculiar way of application, affect the organ of vision; which is illustrated and confirmed by what is by philosophers taught of sound and hearing. Upon which it is inferred, that colours are not any thing permanent in visible things, not of themselves lucid, when they are not illuminated; but that they are the light itself, under some peculiar modification made sensible by the sight.

In a large discourse of the rainbow, its colours and their order, its circular figure, the concentric form of rainbows, &c. he concludes, that a rainbow, both primary and secondary, is generated from the solar rays, reflected and refracted by the drops of a falling cloud, so that the primary is represented by the rays that are once reflected within those drops; but the secondary, by the rays twice reflected, and which after a double refraction in both cases pass to the eye, placed in the axis of the rainbow.

A Letter of Mr. ISAAC NEWTON to the Secretary, containing his New Theory of Light and Colours. — [1672.]

SIR,To perform my late promise to you, I shall without further ceremony acquaint you, that in the beginning of the year 1666 I procured a triangular glass prism, to try therewith the celebrated phenomena of colours. For that purpose having darkened my chamber, and made a small hole in my window shuts, to let in a convenient quantity of the sun's light, I placed my prism at his entrance, that it might be thereby refracted to the opposite wall.

Comparing the length of this coloured spectrum with its breadth, I found it about five times greater; a disproportion so extravagant, that it excited me to a more than ordinary curiosity of examining from whence it might proceed. I could scarce think, that the various thickness of the glass, or the termination with shadow or darkness, could have any influence on light to produce such an effect; yet I thought it not amiss, first to examine those circumstances, and so tried what would happen by transmitting light through parts of the glass of divers thicknesses, or through holes in the window of divers sizes, or by setting the prism without, so that the light might pass through it, and be refracted before it was terminated by the hole; but I found none of those circumstances material. The fashion of the colours was in all these cases the same.

I then proceeded to examine more critically, what might be effected by the difference of the incidence of rays coming from divers parts of the sun; and to that end measured the several lines and angles belonging to the image. Its distance from the hole or prism was 22 feet; its utmost length 131 inches; its breadth 2; the diameter of the hole of an inch; the angle, which the rays, tending towards the middle of the image, made with those lines in which they would have proceeded without refraction, was 44° 56'. And the vertical angle of the prism, 63° 12′. Also the refractions on both sides the prism, that is, of the incident and emergent rays, were as near as I could make them equal, and consequently about 54° 4. And the rays fell perpendicularly upon the wall. Now subducting the diameter of the hole from the length and breadth of the image, there remains 13 inches the length, and 23 the breadth, comprehended by those rays which passed through the centre of the said hole, and consequently the angle of the hole, which that breadth subtended, was about 31', answerable to the sun's diameter; but the angle which its length subtended was more than five such diameters, namely, 2° 49'.

I took two boards, and placed one of them close behind the prism at the window, so that the light might pass through a small hole, made in it for the purpose, and fall on the other board, which I placed at about 12 feet distance, having first made a small hole in it also, for some of that incident light to pass through. Then I placed another prism behind this second board, so that the light, trajected through both the boards, might pass through that also, and be again refracted before it arrived at the wall. This done, I took the first