direction with that of the incident ray, and the other by a vibration in the opposite direction.

A single wave of air or ether would not produce the sensation of sound or light. In order to excite vision, the vibrations of the molecules of ether must be regular, periodical, and very often repeated; and as the ear continues to be agitated for a short time after the impulse, by which alone a sound becomes continuous, so also the fibres of the retina, according to M. d'Arcet, continue to vibrate for about the eighth part of a second, after the exciting cause has ceased. Every one must have observed, when a strong impression is made by a bright light, that the object remains visible for a short time after shutting the eyes, which is supposed to be in consequence of the continued vibrations of the fibres of the retina. Occasionally the retina becomes insensible to feebly illuminated objects when continuously presented. If the eye be turned aside for a moment the object becomes again visible. It is probably on this account that the owl makes so peculiar a motion with its head when looking at objects in the twilight. It is quite possible that many vibrations may be excited in the ethereal medium incapable of producing undulations in the fibres of the human retina, which yet have a powerful effect on those of other animals or of insects. Such may receive luminous impressions of which we are totally unconscious, and at the same time they may be insensible to the light and colours which affect our eyes; their perceptions beginning where ours end.

SECTION XXI.

POLARIZATION OF LIGHT. DEFINED. POLARIZATION BY REFRACTION. PROPERTIES OF THE TOURMALINE.DOUBLE REFRACTION. ALL DOUBLY REFRACTED LIGHT IS POLARIZED. PROPERTIES OF ICELAND SPAR. TOURMALINE ABSORBS ONE OF THE TWO REFRACTED RAYS. UNDULATIONS OF NATURAL LIGHT. UNDULATIONS OF POLARIZED LIGHT. THE OPTIC AXES OF CRYSTALS.-M. FRESNEL'S DISCOVERIES ON THE RAYS PASSING ALONG THE OPTIC AXIS. POLARIZATION BY REFLECTION.

66

IN giving a sketch of the constitution of light, it is impossible to omit the extraordinary property of its polarization," the phenomena of which," Sir John Herschel says, are so singular and various, that to one who has only studied the common branches of physical optics, it is like entering into a new world, so splendid as to render it one of the most delightful branches of experimental enquiry, and so fertile in the views it lays open of the constitution of natural bodies, and the minuter mechanism of the universe, as to place it in the very first rank of the physico-mathematical sciences, which it maintains by the rigorous application of geometrical reasoning its nature admits and requires."

Light is said to be polarized, which, by being once reflected or refracted, is rendered incapable of being again reflected or refracted at certain angles. In general, when a ray of light is reflected from a pane of plate-glass, or any other substance, it may be reflected a second time from another surface, and it will also pass freely through transparent bodies. But if a ray of light be reflected from a pane of plate-glass at an angle of

57°, it is rendered totally incapable of reflection at the surface of another pane of glass in certain definite positions, but it will be completely reflected by the second pane in other positions. It likewise loses the pro

perty of penetrating transparent bodies in particular positions, whilst it is freely transmitted by them in others. Light so modified as to be incapable of reflection and transmission in certain directions, is said to be polarized. This name was originally adopted from an imaginary analogy in the arrangement of the particles of light on the corpuscular doctrine to the poles of a magnet, and is still retained in the undulatory theory.

Light may be polarized by reflection from any polished surface, and the same property is also imparted by refraction. It is proposed to explain these methods of polarizing light, to give a short account of its most remarkable properties, and to endeavour to describe a few of the splendid phenomena it exhibits.

If a brown tourmaline, which is a mineral generally crystallized in the form of a long prism, be cut longitudinally, that is, parallel to the axis of the prism, into plates about the thirtieth of an inch in thickness, and the surfaces polished, luminous objects may be seen through them, as through plates of coloured glass. The axis of each plate is, in its longitudinal section, parallel to the axes of the prism whence it was cut.1 If one of these plates be held perpendicularly between the eye and a candle, and turned slowly round in its own plane, no change will take place in the image of the candle. But if the plate be held in a fixed position, with its axis or longitudinal section vertical, when a

Note 197,

second plate of tourmaline is interposed between it and the eye, parallel to the first, and turned slowly round in its own plane, a remarkable change will be found to have taken place in the nature of the light. For the image of the candle will vanish and appear alternately at every quarter revolution of the plate, varying through all degrees of brightness down to total, or almost total evanescence, and then increasing again by the same degrees as it had before decreased. These changes depend upon the relative positions of the plates. When the longitudinal sections of the two plates are parallel, the brightness of the image is at its maximum; and when the axes of the sections cross at right angles, the image of the candle vanishes. Thus the light, in passing through the first plate of tourmaline, has acquired a property totally different from the direct light of the candle. The direct light would have penetrated the second plate equally well in all directions, whereas the refracted ray will only pass through it in particular positions, and is altogether incapable of penetrating it in others. The refracted ray is polarized in its passage through the first tourmaline, and experience shows that it never loses that property, unless when acted upon by a new substance. Thus, one of the properties of polarized light is proved to be the incapability of passing through a plate of tourmaline perpendicular to it, in certain positions, and its ready transmission in other positions at right angles to the former.

Many other substances have the property of polarizing light. If a ray of light falls upon a transparent medium, which has the same temperature, density, and structure throughout every part, as fluids, gases, glass, &c., and a few regularly crystallized minerals, it is refracted into a single pencil of light by the laws of ordi

nary refraction, according to which the ray, passing through the refracting surface from the object to the eye, never quits a plane perpendicular to that surface. Almost all other bodies, such as the greater number of crystallized minerals, animal and vegetable substances, gums, resins, jellies, and all solid bodies having unequal tensions, whether from unequal temperature or pressure, possess the property of doubling the image or appearance of an object seen through them in certain directions. Because a ray of natural light falling upon them is refracted into two pencils, which move with different velocities, and are more or less separated, according to the nature of the body and the direction of the incident ray. Whenever a ray of natural light is thus divided into two pencils, in its passage through a substance, both of the transmitted rays are polarized. Iceland spar, a carbonate of lime, which, by its natural cleavage, may be split into the form of a rhombohedron, possesses the property of double refraction in an eminent degree, as may be seen by pasting a piece of paper, with a large pin hole in it, on the side of the spar farthest from the eye. The hole will appear double when held to the light. One of these pencils is refracted according to the same law, as in glass or water, never quitting the plane perpendicular to the refracting surface, and is therefore called the ordinary ray. But the other does quit that plane, being refracted according to a different and much more complicated law, and on that account is called the extraordinary ray. For the same reason one image is called the ordinary, and the other the extraordinary image. When the spar is turned round in the same plane, the extraordinary image of the hole

2 Note 198.