does not follow the constant ratio of the sines; r E is therefore called the extraordinary ray, and r O the ordinary ray. In consequence of this bisection

of the light, a spot of ink at O is seen double at O and E, when viewed from r; and when the crystal is turned round, the image E revolves about O, which remains stationary.

NOTE 201. p. 210. Both of the parallel rays Oo and Eo, fig. 63., are polarised on leaving the doubly refracting crystal, and in both the particles of light make their vibrations at right angles to the lines O o, Eo. In the one, however, these vibrations lie, for example, in the plane of the horizon, while the vibrations of the other lie in the vertical plane perpendicular to the horizon.

NOTE 202. p. 211. If light be made to fall in various directions on the natural faces of a crystal of Iceland spar, or on faces cut and polished artificially, one direction A X, fig. 63., will be found, along which the light passes without being separated into two pencils. A X is the optic axis. In some substances there are two optic axes forming an angle with each other. The optic axis is not a fixed line, it only has a fixed direction; for if a crystal of Iceland spar be divided into smaller crystals, each will have its optic axis; but if all these pieces be put together again, their optic axes will be parallel to A X. Every line, therefore, within the crystal parallel to A X is an optic axis; but as these lines have all the same direction, the crystal is still said to have but one optic axis.

NOTE 203. p. 212. If I C, fig. 48., be the incident, and C S the reflected rays, then the particles of polarised light make their vibrations at right angles to the plane of the paper.

NOTE 204. p. 213. Let A B, fig. 48., be the surface of the reflector, IC the incident, and C S the reflected rays; then, when the angle S C B is 57°, and consequently the angle P C S equal to 33°, the black spot will be seen at C by an eye at S.

NOTE 205. p. 213. Let A B, fig. 48., be a reflecting surface, IC the incident, and C S the reflected rays; then, if the surface be plate glass, the angle S C B must be 570, in order that CS may be polarised. If the surface be crown glass or water, the angle S C B must be 56° 55′ for the first, and 53° 11′ for the second, in order to give a polarised ray.

NOTE 206. p. 215. A polarising apparatus is represented in fig. 64., where Rr is a ray of light falling on a piece of glass r at an angle of 57°, the reflected

ray r s is then polarised, and may be viewed through a piece of tourmaline in s, or it may be received on another plate of glass, B, whose surface is at right angles to the surface of r. The ray r s is again reflected in s, and comes to the eye in the direction s E. The plate of mica, M I, or of any substance that is to be examined, is placed between the points r and s.

NOTE 207. p. 217. In order to see these figures, the polarised ray r s, fig. 64., must pass through the optic axis of the crystal, which must be held as near as possible to s on one side, and the eye placed as near as possible to s on the other. Fig. 65. shows the image formed by a crystal of Iceland spar which has one optic axis. The colours in the rings are exactly the same with those of Newton's rings given in Note 194., and the cross is black. If the spar be turned round its axis, the rings suffer no change; but if the tourmaline through which it is viewed, or the plate of glass B, be turned round, this figure will be seen at the angles 00, 900, 1800, and 270° of its revolution. But in the intermediate points, that is, at the angles 45°, 1350, 2250, and 3150, another system will appear, such as represented in fig. 66., where all the colours of the rings are complementary to those of fig. 65., and the cross is white. The two systems of rings, if superposed, would produce white light.

NOTE 208. p. 217. Saltpetre, or nitre, crystallises in six-sided prisms having two optic axes inclined to one another at an angle of 50. A slice of this substance about the 6th or 8th of an inch thick, cut perpendicularly to the axis of the prism, and placed very near to s," fig. 64., so that the polarised ray r s

may pass through it, exhibits the system of rings represented in fig. 67., where the points C and C mark the position of the optic axes.

Fig. 67.

When the plate B, Fig. 68.

fig. 64., is turned round, the image changes successively to those given in figs. 68, 69, and 70. The colours of the rings are the same with those of thin plates, but they vary with the thickness of the nitre. Their breadth enlarges or diminishes also with the colour, when homogeneous light is used.

Fig. 69.

Fig. 70.

NOTE 209. p. 219. Fig. 71. represents the appearance produced by placing

a slice of rock crystal in the polarised ray rs, fig. 64. The uniform colour in

NOTE 210. p. 223. Suppose the major axis A P of an ellipse, fig. 18., to be invariable, but the excentricity CS continually to diminish, the ellipse would bulge more and more; and when C S vanished, it would become a circle whose diameter is A P. Again, if the excentricity were continually to increase, the ellipse would be more and flattened till C S was equal to C P, when it would become a straight line A P. The circle and straight line are therefore the limits of the ellipse.

NOTE 211. p. 224. The coloured rings are produced by the interference of two polarised rays in different states of undulation, on the principle explained for common light.

NOTE 212. p. 244. If heat from a non-luminous source be polarised by reflection or refraction at r, fig. 64., the polarised ray rs will be stopped or transmitted by a plate of mica M I under the same circumstances that it would stop or transmit the light; and if heat were visible, images analogous to those of figs. 65., 67., &c., would be seen at the point s.

NOTE 213. p. 247. The Rev. John Buchanan, of Charleston, South Carolina, has recently shown, by ingenious experiments, that the vulture is directed to his prey by the sense of sight alone.

NOTE 214. p. 296. The class Cryptogamia contains the ferns, mosses, funguses, and sea-weeds: in all of which the parts of the flowers are either little known, or too minute to be evident.

NOTE 215. p. 298. Zoophites are the animals which form madrepores, corals, sponges, &c.

NOTE 216. p. 299. The Saurian tribes are creatures of the lizard or crocodile kind. Some of those found in a fossil state are of enormous size.

NOTE 217. p. 347. When a stream of positive electricity descends from P to n, fig. 72., in a vertical wire at right angles to the plane of the horizontal circle A B, the negative electricity ascends from n to P, and the force exerted by the current makes the north pole of a magnet revolve about the wire in the direction of the arrow heads in the circumference, and it makes the south pole revolve in the opposite direction. When the current of positive electricity flows upwards from n to P,

Fig. 72.

Fig. 75.

NOTE 218. p. 349. Fig. 73. represents a helix or coil of copper wire, terminated by two cups containing a little quicksilver. When the positive wire of a Voltaic battery is immersed in the cup P, and the negative wire in the cup n, the circuit is completed. The quicksilver insures the connection between the battery and the helix, by conveying the electricity from the

one to the other. While the electricity flows through the helix, the magnet S N remains suspended within it, but falls down the moment it ceases. The magnet always turns its south pole S towards P the positive wire of the battery, and its north pole towards the negative wire.

NOTE 219. p. 352. A copper wire coiled in the form represented in fig. 73. is an electro-dynamic cylinder. When its extremities P and n are connected with the positive and negative poles of a Voltaic battery, it becomes a perfect magnet during the time that a current of electricity is flowing through it, P and n being its north and south poles. There are a variety of forms of this apparatus.

NOTE 220. p. 377. In fig. 74. the hyperbola HP Y, the parabola p P R, and

the ellipse A EPL have the same focal distance S P, and coincide through a small space on each side of the perihelion P; and as a comet is only visible when near P, it is difficult to ascertain in which of the three curves it moves.

NOTE 221. p. 381. In fig. 75., E A represents the orbit of Halley's comet, ET the orbit of the earth, and S the sun. The proportions are very nearly

exact.

Fig. 75.

T

A