Transparent bodies, particularly fluids, often transmit light of one colour and reflect light of another. Halley told Newton, that, being deep under the surface of the sea in a diving-bell, in a clear sunshine day, the upper side of his hand, on which the sun shone darkly through the water, and through a small glass window in the diving-bell, appeared of a red colour, like a damask rose, while the water below, and the under part of his hand, looked green.' But, in explaining the permanent colour of bodies, this difficulty always presents itself,—Suppose that a body reflects red or green light, what is it that decomposes the light, and separates the red or the green from the rest ? Refraction is the only means of decomposing light, and separating the rays of one degree of refrangibility and of one colour, from those of another. This appears to have been what led Newton to study the colours produced by light passing through thin plates of any transparent substance. The appearances are very remarkable, and had already attracted the atten tion, both of Boyle and of Hook, but the facts observed by them remained insu lated in their hands, and unconnected with other optical phenomena.
It probably had been often remarked, that when two transparent bodies, such as glass, of which the surfaces were convex in a certain degree, were pressed together, a black spot was formed at the contact of the two, which was surrounded with coloured rings, more or less regular, according to the form of the surfaces. In order to ana the experiment with surfaces of a regular curvature, such as was capable of being measured. He took two object glasses, one a plano-convex for a fourteen feet tele scope, the other a double convex for one of about fifty feet, and upon this last he laid the other with its plane side downwards, pressing them gently together. At their contact in the centre was a pellucid spot, through which the light passed without suf fering any reflection. Round this spot was a coloured circle or ring, exhibiting blue, white, yellow, and red. This was succeeded by a pellucid or dark ring, then a coloured ring of violet, blue, green, yellow, and red, all copious and vivid except the green. The third coloured ring consisted of purple, blue, green, yellow, and red. The fourth consisted of green and red; those that succeeded became gradually more dilute and ended in whiteness. It was possible to count as far as seven.
The colours of these rings were so marked by peculiarities in shade and vivacity, that Newton considered them as belonging to different orders ; so that an eye ac customed to examine them, on any particular colour of a natural object being pointed out, would be able to determine to what order in this series it belonged.
Thus we have a system of rings or zones surrounding a dark central spot, and them selves alternately dark and coloured, that is, alternately transmitting the light and re flecting it. It is evident that the thickness of the plates of air interposed between the glasses, at each of those rings, must be a very material element in the arrangement of this system. Newton, therefore, undertook to compute their thickness. Having care fully measured the diameters of the first six coloured rings, at the most lucid part of each, he found their squares to be as the progression of odd numbers 1, 3, 1, &e. The squares of the distances from the centre of the dark spot to each of these circum ferences, were, therefore, in the same ratio, and consequently the thickness of the plates of air, or the intervals between the glasses, were as the numbers 3, 5, 7, &c.
When the diameters of the dark or pellucid rings which separated the coloured rings were measured, their squares were found to be as the even numbers 0, e, 4, 6, and, therefore, the thickness of the plates through which the light was wholly trans .
mitted were as the same numbers. A great many repeated measurements assured the accuracy of these determinations.
As the curvature of the convex glass on which the flat surface of the plano-convex rested was known, and as the diameters of the rings were measured in inches, it was easy to compute the thickness of the plates of air, which corresponded to the different, rings.
An inch being divided into 178000 parts, the distance of the lenses for the first se When the rings were examined by looking through the lenses in the opposite di rection, the central spot appeared white, and, in other rings, red was opposite to blue, yellow to violet, and green to a compound of red and violet ; the colours formed by the transmitted and the reflected light being, what is now called, complementary, or nearly so, of one another ; that is, such as when mixed produce white.
When the fluid between the glasses was different from air, as when it was water, the succession of rings was the same ; the only difference 'was, that the rings them selves were narrower.
When experiments on thin plates were made in such a way that the plate was of a denser body than the surrounding medium, as in the case of soap-bubbles, the same phenomena were observed to take place. phenomena Newton also examined with his accustomed accuracy, and even bestowed particular care on having the soap bubbles as perfect and durable as their frail structure would admit. In the eye of phi losophy no toy is despicable, and no occupation frivolous, that can assist in the dis covery of truth.