blown glass, the bands will have no regular shape, the same thickness giving always the same colour.

The preceding doctrine of fits has always been regarded as an ingenious explanation of the colours of thin plates. It is not given by its author as a theory or a hypothesis, but simply as an expression of the facts which he has observed; and yet it has to a certain extent the character of a hypothesis, in so far as it assumes that the second surface of the plate does not in every part of it reflect light like the first, whereas in the theory of interference, certain portions thus reflected are destroyed before they reach the eye of the observer.

With the exception of the interesting observations of MM. Provostayes and Desains already referred to, no discovery of any great importance has been made on the subject of thin plates since the time of Newton. We have had occasion to observe a number of curious phenomena in the thin plates of decomposed glass when acting upon light in a state of combination. The colours which they reflect and transmit are not deducible from any theory of light, and have an intimate connexion with the absorption of light by coloured media.1

Among natural phenomena illustrative of the colours of thin plates, we have found none more remarkable than one exhibited by the fracture of a large crystal of quartz of a smoky colour, and about two and a quarter inches in diameter. The surface of fracture, in place of being a face of cleavage, or irregularly conchoidal, as we have sometimes seen it, was filamentous like a surface of velvet, and consisted of short fibres so small as to be incapable of reflecting light. Their size could not have been greater than

"On the Connexion between the Phenomena of the Absorption of Light and the Colours of Thin Plates."-Phil. Trans. 1837, p. 245.

the third of the millionth part of an inch, or one-fourth of the thinnest part of the soap-bubble when it exhibits the black spot where it bursts.1

Although Newton did not communicate his observations on the colours of thick plates to the Royal Society in hist discourse on light and colours, but "put them together out of scattered papers" some time before the publication of his Optics,2 yet this is the proper place for bringing them under the notice of the reader.

The colours of thick plates arise from a quantity of light scattered in all directions from the little inequalities or imperfections which exist in the surface of a glass mirror either silvered or unsilvered. In order to observe them, a sunbeam is admitted through a small hole about a third of an inch in diameter into a dark room. This beam is received perpendicularly on a concavo-convex glass mirror, a quarter of an inch thick, and having each surface ground to a sphere six feet in radius. When the sunbeam passes through a small hole in the middle of a sheet of white paper placed in the centre of the mirror's concavity, the hole is surrounded with four or five coloured rings. The rings resembled those seen by transmission through two object-glasses, but were larger and fainter in their colours. When mirrors of different thicknesses were used, the diameters of the rings were reciprocally as the square roots of the thicknesses; and in homogeneous light they were largest in the red, and smallest in the violet rays, like those formed by thin plates.

}

These and other phenomena described by Newton, he

Edinburgh Journal of Science, vol. i. p. 108. June 1824.

2 These observations, thirteen in number, entitled "Observations concerning the Reflections and Colours of thick transparent polished Plates, form the fourth part of the Second Book of Optics.

explains by taking into consideration the fits of easy reflexion and transmission of the faint scattered light already mentioned. On the undulatory theory they are explained by the interference of the portions of light scattered at the first surface by the rays in passing and repassing through it.

The Duke de Chaulnes1 observed similar rings when the surface of the mirror was covered with fine gauze, or with a thin film of milk dried upon it, and Sir William Herschel 2 noticed analogous colours when hair-powder was scattered in the air before a metallic mirror, on which a beam of light was incident.

When we look through two plates of parallel glass of exactly the same thickness, at a circular disc of light 1° or 2° in diameter, no coloured bands will be seen when the light is incident perpendicularly, and when the plates are parallel. But if we incline them slightly to one another, we shall see, beside the direct image of the luminous body which is crossed with no fringes, a series of lateral images formed by successive reflexions between the surfaces of the plates, which are crossed with fifteen or sixteen highly coloured bands parallel to the common section of the surfaces of the plates. The breadth of these bands is inversely as the inclination of the plates, and at a given inclination their magnitudes are inversely as the thickness of the plates employed.

3

These brilliant bands, which we have described minutely in a separate memoir, are explicable by the doctrine of fits of easy reflexion and transmission. They have been explained also on the undulatory hypothesis by Dr.

1 Mém. Acad. Par. 1705.

2 Phil. Trans. 1807.

Edinburgh Transactions, 1815, vol. vii. p. 435.

Thomas Young,1 and in greater detail by Sir John Herschel.2

Another species of coloured fringes, produced by the reflexion of a pencil of light between the lenses of a double or a triple achromatic object-glass, is equally explicable by Newton's theory of fits, and by the doctrine of interference. Owing to the curvature of the surfaces which produce them, the forms of the isochromatic lines, or the lines of equal tint, are various and beautiful. 3

1 Art. CHROMATICS in Encyclopædia Britannica.

2 Treatise on Light, 2 688-695.

3 Edinburgh Transactions, 1832, vol. xii.

CHAPTER VIII.

INFLUENCE OF COLOUR IN THE MATERIAL WORLD-NEWTON'S THEORY OF THE COLOURS OF NATURAL BODIES-COLOURED BODIES REFLECT ONLY LIGHT OF THEIR OWN COLOUR, ABSORBING ALL THE OTHER PARTS OF WHITE LIGHT THE COLOURS OF NATURAL BODIES ARE THOSE OF THIN PLATES-THE TRANSPARENT PARTS REFLECTING ONE COLOUR AND TRANSMITTING ANOTHER-ARRANGEMENT OF THE COLOURS EXHIBITED IN NATURAL BODIES INTO SEVEN CLASSES-COLOURED JUICES AND SOLUTIONS, OXIDATED FILMS, METALS, ETC. ETC.-NEWTON'S THEORY APPLICABLE ONLY TO ONE CLASS OF COLOURS-OBJECTIONS TO IT STATED-MR. JAMIN'S RESEARCHES ON THE COLOURS OF METALSCAUSE OF COLOURS MUST BE IN THE CONSTITUTION OF BODIES-EXAMPLES OF THE EFFECT OF HEAT UPON RUBIES AND NITROUS GASEFFECT OF SUDDEN COOLING-ON PHOSPHORUS-EFFECT OF MECHANICAL ACTION ON IODIDE OF MERCURY-INDICATION OF A NEW THEORY ·AND OF THE CAUSE OF THE ABSORPTION OF DEFINITE RAYS-ILLUSTRATION OF THESE VIEWS IN A REMARKABLE TOURMALINE.

HAD the objects of nature been rendered visible only by white light, and exercised upon it the same action in refracting and reflecting it to the human eye, all the combinations in the material world, and all the various forms of life, would have displayed no other tint than that which they exhibit in a pencil sketch, a China-ink drawing, or a photographic picture. The magnificent foliage of the vegetable world might have filled the eye with its picturesque and lovely forms, and given protection to its fruit and its flowers, but we should not have rejoiced in the verdure of its youth, nor mourned over the yellow of its age. The sober mantle of twilight would have replaced the golden vesture of the rising and the setting sun. The