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 1/178000, 3/178000, 5/178000, 7/178000, 9/178000, 11/178000. 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 1/1·336 of the thickness at which they are produced in air, and in the case of glass or mica at 1/1·525 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 hypothetical, 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.