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CHAPTER VI.

Coloura of thin Plates first studied by Boyle and Hooke-Newton de

termines the Law of their Production-His Theory of Fits of Easy Reflection and Transmission-Colours of thick Plates.

In examining the nature and origin of colours as the component parts of white light, the attention of Newton was directed to the curious subject of the colours of thin plates, and to its application to explain the colours of natural bodies. His earliest researches on this subject were communicated, in his Discourse on Light and Colours, to the Royal Society, on the 9th December, 1675, and were read at subsequent meetings of that body. This discourse contained fuller details respecting the composition and decomposition of light than he had given in his letter to Oldenburg, and was concluded with nine propositions, showing how the colours of thin transparent plates stand related to those of all natural bodies.

The colours of thin plates seem to have been first observed by Mr. Boyle. Dr. Hooke afterward studied them with some care, and gave a correct account of the leading phenomena, as exhibited in the coloured rings upon soap-bubbles, and between plates of glass pressed together. He recognised that the colour depended upon some certain thickness of the transparent plate, but he acknowledges that he had attempted in vain to discover the relation between the thickness of the plate and the colour which it produced.

Dr. Hooke succeeded in splitting a mineral substance, called mica, into films of such extreme thinness as to give brilliant colours. One plate, for ex

ample, gave a yellow colour, another a blue colour, and the two together a deep purple ; but, as plates which produced those colours were always less than the 12,000th part of an inch thick, it was quite impracticable, by any contrivance yet discovered, to measure their thickness, and determine the law according to which the colour varied with the thickness of the film. Newton surmounted this difficulty by laying a double convex lens, the radius of curvature of each side of which was fifty feet, upon the flat surface of a plano-convex object-glass, and in this way he obtained a plate of air or of space varying from the thinnest possible edge at the centre of the object-glass where it touched the plane surface, to a considerable thickness at the circumference of the lens. When light was allowed to fall upon the object-glass, every different thickness of the plate of air between the object-glass gave different colours, so that the point where the two object-glasses touched one another was the centre of a number of concentric coloured rings. Now, as the curvature of the object-glass was known, it was easy to calculate the thickness of the plate of air at which any particular colour appeared, and thus to determine the law of the phenomena.

In order to understand how he proceeded, let CED be the convex surface of the one object-glass, and AEB the flat surface of the other. Let them touch at the point E, and let homogeneous red rays fall upon them, as shown in the figure. At the point of contact E, where the plate of air is inconceivably thin, not a single ray of the pencil RE is reflected. The light is wholly transmitted, and, consequently, to an eye above E, there will appear at E a black spot. At a, where the plate of air is thicker, the red light ra is reflected in the direction aa', and as the air has the same thickness in a circle round the point E, the eye above E, at a, will see next the black spot E ring of red light. At m,

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where the thickness of the air is a little greater than at a, the light rm is all transmitted as at E, and not a single ray suffers reflection, so that to an eye above E at m' there will be seen without the red ring a a dark ring m. In like manner, at greater thicknesses of the plate of air, there is a succession of red and dark rings, diminishing in breadth as shown in the diagram.

When the same experiment was repeated in orange, yellow, green, blue, indigo, and violet light, the very same phenomenon was observed; with this difference only, that the rings were largest in red light, and smallest in violet light, and had intermediate magnitudes in the intermediate colours.

If the observer now places his eye below E, so as to see the transmitted rays, he will observe a set of rings as before, but they will have a bright spot in their centre at E, and the luminous rings will now correspond with those which were dark when seen by reflection, as will be readily understood from inspecting the preceding diagram. When the object-glasses are illuminated by white

light, the seven systems of rings, formed by all the seven colours which compose white light, will now be seen at once. Had the rings in each colour been all of the same diameter they would all have formed brilliant white rings, separated by dark intervals ; but, as they have all different diameters, they will overlap one another, producing rings of various colours by their mixture. These colours, reckoning from the centre E, are as follows:

Ist Order. Black, blue, white, yellow, orange, red. 2d Order. Violet, blue, green, yellow, orange, red.

3d Order. Purple, blue, green, yellow, red, bluishred.

4th Order. Bluish-green, green, yellowish-green, red.

5th Order. Greenish-blue, red. 6th Order. Greenish-blue, red.

By accurate measurements, Sir Isaac found that the thicknesses of air at which the most luminous parts of the first rings were produced, were in parts of an inch TOOTTito, ito roti TT TOT, TYÖTOTI Tyy. If the medium or the substance of the thin plate is water, as in the case of the soap-bubble, which produces beautiful colours according to its different degrees of thinness, the thicknesses at which the most luminous parts of the rings appear are produced at 1.75m of the thickness at which they are produced in air, and in the case of glass or mica at ze of that thickness; the numbers 1.336, 1.525 expressing the ratio of the sines of the angles of incidence and refraction in the substances which produce the colours.

From the phenomena thus briefly described, Sir Isaac Newton deduces that ingenious, though hy. pothetical, property of light, called its fits of easy reflection and transmission. This property consists in supposing that every particle of light from its first discharge from a luminous body possesses, at equally distant intervals, dispositions to be reflected from, and transmitted through, the surfaces of bodies upon which it is incident. Hence, if a particle of light reaches a reflecting surface of glass when it is in its fit of reflection, or in its disposition to be reflected, it will yield more readily to the reflecting force of the surface; and, on the contrary, if it reaches the same surface while in a fit of easy transmission, or in a disposition to be transmitted, it will yield with more difficulty to the reflecting force. Sir Isaac has not ventured to inquire into the cause of this property; but we may form a very intelligible idea of it by supposing, that the particles of light have two attractive and two repulsive poles at the extremities of two axes at right angles to each other, and that the particles revolve round their axes, and at equidistant intervals bring one or other of these axes into the line of the direction in which the particle is moving. If the attractive axis is in the line of the direction in which the particle moves when it reaches the refracting surface, the particle will yield to the attractive force of the medium, and be refracted and transmitted ; but if the repulsive axis is in the direction of the particle's motion when it reaches the surface, it will yield to the repulsive force of the medium, and be reflected from it.

The application of the theory of alternate fits of reflection and transmission to explain the colours of thin plates is very simple. When the light falls upon the first surface AB, Fig. 8 of the plate of air between AB and CED, the rays that are in a fit of reflection are reflected, and those that are in a fit of transmission are transmitted. Let us call F the length of a fit, or the distance through which the particle of light moves while it passes from the state of being in a fit of reflection to the state of being in a fit of transmission. Now, as all the particles of light transmitted through AB were in a state of easy transmission when they entered AB, it is obvious, that, if the plate of air at E is so thin as to be less * This ayud in each at right angle

with the other than intammed.

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