of air corresponding to each color, from the breadth of the rings, which are always of the same color with the homogeneous light.

NOTE 196, p. 168.-The focal length or distance of a lens is the distance from its center to the point F. fig. 60, in which the refracted rays meet. Let LL be a lens of very short focal distance fixed in the window-shutter of a dark room. A sunbeam SL L', passing through the lens, will be brought to a focus in F, whence it will diverge in lines FC, FD, and will form a circular image of light on the opposite wall. Suppose a sheet of lead, having a small pin-hole pierced through it, to be placed in this beam; when the pin-hole is viewed from behind with a lens at E, it is surrounded with a series of colored rings, which vary in appearance with the relative positions of the pin-hole and eye with regard to the point F. When the hole is the 30th of an inch in diameter and at the distance of 64 feet from F, when viewed at the distance of 24 inches, there are seven rings of the following colors :

1st order: White, pale yellow, yellow, orange, dull red.

2d order: Violet, blue, whitish, greenish yellow, C fine yellow, orange red.

3d order: Purple, indigo, blue, greenish blue, brilliant green, yellow green, red.

4th order: Good green, bluish white, red.

5th order: Dull green, faint bluish white, faint red.

6th order: Very faint green, very faint red. 7th order: A trace of green and red.

H

E

Fig. 60.

NOTE 197, p. 168.-Let L L', fig. 61, be the section of a lens placed in a window-shutter, through which a very small beam of light SLL' passes into a dark room, and comes to a focus in F. If the edge of a knife K N be held in the beam, the rays bend away from it in hyperbolic curves Kr, Kr', &c. instead of coming directly to the screen in the straight line K E, which is the boundary of the shadow. As these bending rays arr.ve at the screen in different states of undulation, they interfere, and form a series of colored fringes, rr, &c. along the edge of the shadow KESN of the knife. The fringes vary in breadth with the relative distances of the knife-edge and screen from F.

S

Fig. 61.

NOTE 198, p. 171. Fig. 43 represents the phenomena in question, where SS is the surface, and I the center of incident waves. The reflected waves are the dark lines returning toward I, which are the same as if they had originated in C on the other side of the surface.

NOTE 199, p. 173. Fig. 62 represents a prismatic crystal of tourmaline, whose axis is AX. The slices that are used for polarizing light are cut parallel to AX.

Fig. 62.

Fig. 63.

M

B

R

NOTE 200, p. 175.-Double refraction. If a pencil of light, Rr, fig. 63, falls upon a rhombohedron of Iceland spar, AB XC, it is separated into two equal pencils of light at r, which are refracted in the directions rO, rE: when these arrive at O and E they are again refracted, and pass into the air in the directions Oo, Eo, parallel to one another and to the incident ray Rr. The ray rO is refracted according to the ordinary law, which is, that the sines of the angles of incidence and refraction bear a constant ratio to one another (see Note 184), and the rays Rr, rO, O. are all in the same plane. The pencil r E, on the contrary, is bent aside out of that plane, and its refraction does not follow the constant_ratio of the sines; r E is therefore called the extraordinary ray, and rO 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. 176. Both of the parallel rays Oo and Eo, fig. 63, are polarized on leaving the doubly refracting crystal, and in both the particles of light make their vibrations at right angles to the lines Oo 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. 177. 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. AX 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 AX. Every line, therefore, within the crystal parallel to AX 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. 178. If IC, fig. 48, be the incident and CS the reflected

rays, then the particles of polarized light make their vibrations at right angles to the plane of the paper.

NOTE 204, p. 178. Let A B, fig. 48, be the surface of the reflector, IC the incident, and CS the reflected rays; then, when the angle SCB is 570, and consequently the angle PCS equal to 330, the black spot will be seen at C by an eye at S.

NOTE 205, p. 179. Let A B, fig. 48, be a reflecting surface, IC the incident, and CS the reflected rays; then, if the surface be plate-glass, the angle SCB must be 570, in order that CS may be polarized. If the surface be crown-glass or water, the angle SCB must be 560 55' for the first, and 530 11' for the second, in order to give a polarized ray.

NOTE 206, p. 180. A polarizing apparatus is represented in fig. 64, where Rr is a ray of light falling on a piece of glass r at an angle of 570,

the reflected ray rs is then polarized, 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, MI, or of any substance that is to be examined, is placed between the points r and s.

NOTE 207, p. 182. In order to see these figures, the polarized ray rs, 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 colors 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 2700 of its revolution. But in the intermediate points, that is, at the angles 450, 1350, 2250, and 3150, another system will appear, such as is represented in fig. 66, where all the colors of the Fig. 65.

Fig. 66.

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. 182. Saltpetre, or nitre, crystalizes 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 polarized 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 colors 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 color, when homogeneous light is used.

NOTE 209, p. 183. Fig. 71 represents the appearance produced by placing a slice of rock crystal in the polarized ray rs, fig. 64. The uniform color in the interior of the image depends upon the thickness of the slice; but whatever that color 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.

Fig. 71.

NOTE 210, p. 187. Suppose the major axis A P of an ellipse, fig. 18, to be invariable, but the eccentricity C S continually to diminish, the ellipse would bulge more and more; and when CS vanished, it would become a circle whose diameter is A P. Again, if the eccentricity were continually to increase, the ellipse would be more and more flattened till CS was equal to CP, when it would, become a straight line A P. The eircle and straight line are therefore the limits of the ellipse.

NOTE 211, p. 187.-The colored rings are produced by the interference of two polarized rays in different states of undulation, on the principle explained for common light.

NOTE 212, p. 217.-If heat from a non-luminous source be polarized by reflection or refraction at r, fig. 64, the polarized ray rs will be stopped or transmitted by a plate of mica MI under the same circumstances that it would stop or transinit 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. 219.-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. 267.-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. 269.-Zoophites are the animals which form madre pores, corals, sponges, &c.

NOTE 216, p. 269.-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. 315.-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 A wire in the direction of the arrowheads in the circumference, and it makes the south pole revolve in the opposite direction. When the current of positive electricity flows upward from n to P, these effects are reversed.

P

Fig. 72.