law of refraction discovered by Snellius, it is called the ordinary pencil or the ordinary image, while the other pencil or image is called the extraordinary one, from its being refracted according to a law different from the ordinary law.

The light which composes the two images or pencils thus formed by double refraction, possesses properties different from all ordinary light, and as these properties are related to the different sides of the rays or pencils, the rays or pencils of light which possess such properties, are said to be polarised, or to indicate polarity, in the same way as particles of iron in the vicinity of a magnet indicate polarity, by possessing a property in one of their sides or extremities, which they do not possess in their other extremity. Now, as the diameter of a beam of light may be reduced to a very small magnitude, and as every portion of the beam in the direction of its length has a progressive motion, and possesses, the same properties as the whole beam, we may call these minute portions Particles of Light, and speak of the Sides or Poles of these particles, without considering whe ther light is composed of material particles issuing from the sun's body, or is merely the undulation of an elastic medium.

PERIOD I. Account of the Discoveries of Erasmus Bartholinus, respecting Double Refraction.

About the middle of the seventeenth century, Dr Erasmus Bartholinus, a physician at Copenhagen, and the author of several excellent works on geometry, procured from some of the Danish merchants that frequented Iceland, "a crystal stone like a rhombick or rhomboid prism, which, when broken into small pieces, kept the same figure." It was dug out of a very high mountain, not far from the Bay of Roerfiord, in 65° of latitude. "Its whole body was rather clear than bright, of the colour of limpid water, but that colour, when it was immersed in water and dried again, became dull." With this substance, which, from its locality, was called Iceland Crystal, Bartholinus made a number of experiments both chemical and optical; and having discovered some of the singular effects which it produced upon light, he published an account of them at Copenhagen in 1669, under the title of Experimenta Crystalli Islandici Dis-diaclastici, quibus mira et insolita refractio detegitur. There does not ap

pear to be a copy of this work in this country, and we have learned that it is not to be found in the library of the Royal Society; but the want of it is well supplied by "An account of sundry experiments made and communicated by Dr Erasmus Bartholin, upon a crystal-like body sent to him out of Iceland," addressed to Dr Oldenburgh, and printed in the 67th Number of the Philosophical Transactions. From this account we shall select those parts which relate to double refraction, and shall in general give them in the words of the author.

"1. The objects seen through" this crystalline prism "appear sometimes, and in certain positions of the prism, double; where 'tis to be noted, that the distance between the two images is greater or less, according to the different bigness of the prism; insomuch, that in thinner pieces this difference of the double image almost vanisheth.

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

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

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

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

"6. If any of the obtuse angles of this prism be divided into two equal parts by a line, and the visual rays do 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.

"7. 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 moveable, the other remaining fixed; although there be a way also to make the fixed image moveable,

The sextuple multiplication of the image here mentioned, arises, as will be seen in a subsequent article, from thin contemporaneous veins of Iceland spar intersecting the rhomboidal mass. See Phil. Trans. 1816, p. 270; and Edin. Trans. vol. viii. p. 165.

and the moveable fixed in the same crystal, and another to make both moveable.

"8. The moveable image doth not move at random, but always about the fixed, which, while it turneth about, it never describeth a perfect circle but in one case.

"9. Dioptricks teach, that a diaphanous body having one only surface, sends from one object but one image refracted to the eye, and, having more surfaces than one, it represents one image in each. But, whereas in our substance there occurs but one plain superficies to the eye, and yet a double image of one object, it concerned us to consider whence this double image might be caused. Two ways offered themselves to us, reflection and refraction. How reflection could perform it, was difficult to find. For, having dulled the clearness of the two plain sides of our crystalline prism, thereby to make them unfit for reflecting the light, the rays being directed through its upper and lowermost superficies, the image still appeared double. Again, two species appearing through a great prism, upon breaking of the same into pieces, and so reducing it into divers smaller ones, it came to pass, that through each of these lesser portions the same object was seen always double. Whence I collected, that if it should be said that one of the images proceeded from the reflection of the plain sides, the former of these experiments would discountenance that assertion. But then if another should derive the cause from some internal reflection of the surfaces of this body, certainly the same effect would not have been found in every one of its parts; but the double appearance that was exhibited in the smallest portion, would have been multiplied in a greater bulk.

"Reflection, therefore, not satisfying, we recurred to refrac tion. But whereas 'tis known, that no image can pass through two diaphanous bodies of a different nature but by refraction, and that one image supposeth one refraction, it did follow, that, if refraction were made the cause of this phenomenon, there would be a double refraction for a double image. And forasmuch as the appearances of our Iceland crystal are not of the same kind, but one of them is fixed, the other moveth, we shall distinguish the refractions themselves, which refract the double rays arriving to the eye, and call the one which sends the fixed image refracted to our sight, Usual; the other, which transmits