the incident, and CS the reflected rays; then, when the angle S C B is 570, and consequently the angle P C S equal to 330, the black spot will be seen at C by an eye at S.

NOTE 203. p. 220. 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 C S 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 204. p. 222. A polarising apparatus is represented in fig. 64.,

Fig. 64.
M

I

E

B

R

where R r is a ray of light falling on a piece of glass r at an angle of 59 4 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 rs 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 205. p. 224. In order to see these figures, the polarised rays, 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 Ice

land spar which has one optic axis. The colours in the rings are exactly the same with those of Newton's rings given in Note 192, 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 0°, 90°, 180°, and 270° of its revolution. But in the intermediate points, that is, at the angles 450, 1350, 225°, and 315o, 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 black cross is white. The two systems of rings, if superposed, would produce white light.

NOTE 206. p. 224. 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 rs 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. When the plate B, 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. 71.

NOTE 207. p. 226. Fig. 71. represents the appearance produced by placing a slice of rock crystal in the polarised ray r s, fig. 64. The uniform colour in the interior of the image depends upon the thickness of the slice; but whatever that colour may be, it will alternately attain a maximum brightness and vanish with the revolution of the glass B. It may be observed, that the two kinds of quartz, or rock crystal, mentioned in the text, are combined in the amethyst, which consists of alternate layers of right-handed and left-handed quartz, whose planes are parallel to the axis of the crystal.

NOTE 208. p. 230. Suppose the major axis A P of an ellipse, fig. 81., 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 more flattened till C S was equal to CP, when it would become a straight line A P. The circle and straight line are therefore the limits of the ellipse.

NOTE 209. p. 231. 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 210. p. 264. A mirror is a polished metallic surface, which may be

NOTE 211. p. 294. 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 212. p. 297. Zoophites are the animals which form madrepores, corals, sponges, &c.

NOTE 213. p. 297. The Saurian tribes are creatures of the lizard or crocodile kind. Some of those found in a fossil state are of enormous size.

NOTE 214. p. 344. 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 A 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, these effects are reversed.

Fig. 72.

n

NOTE 215. p. 346. Fig. 73. represents a helix or coil of coppe wire, terminated by two cups con

Fig. 73.

ing 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 216. p. 351. 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 be. comes 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 217. p. 404. One of the globular clusters mentioned in the text, is represented in fig. 1. plate 5. The stars are gradually condensed towards the centre, where they run together into a blaze somewhat like a snowball. The more condensed part is projected on a ground of irregularly scattered stars, which fills the whole field of the telescope. There are few stars in the neighbourhood of this cluster.

NOTE 218. p. 406.

Fig. 2. plate 5. represents one of those enormous rings in its oblique position. It has a dark space in the centre, with a small star at each extremity.

NOTE 219. p. 407. Fig. 3. plate 5. may convey some idea of the ring in the constellation of the Lyre mentioned in the text.

NOTE 220. p. 407. This most wonderful object has the appearance of fig. 4. plate 5. The southern head is denser than the northern. The light of this object is perfectly milky There are one or two stars in it.

NOTE 221. p. 407. Fig. 5. plate 5. represents this brother system.

NOTE 222. p. 408. Fig. 6. plate 5. represents one of the spindle-shaped nebulæ.