Sir Isaac Newton also discovered that when the sun's light is reflected into a darkened room, and allowed to fall on a screen with a moderately small hole, and the beam so pegging is received perpendicu larly on a concave mirror, made of glass quicksilvered at the back, which is placed at such a distance from the screen that the hole coin cides with the centre of curvature of its surfaces, so that the regularly reflected light goes back through the same small hole by which it entered, a system of coloured rings is seen depicted on the screen, on the face towards the mirror, surrounding the hole. The order of colours, and the law of the diameters of the rings, agree with the transmitted system of the rings formed between two object-glasses. A metallic speculum exhibits no such rings. If the amalgam be removed from a mirror of quicksilvered glass, the rings are seen as before, but much fainter. With different mirrors the diameter of a given ring varies directly as the radius of curvature of the surfaces, and inversely as the square root of the thickness of the glass.
Although Newton expressly refers to the defect of polish of the first surface in relation to these rings, ho does not seem to have purposely tarnished his mirrors: In repeating Newton's experiments, the Duke de Chaulnes observed that the brilliancy of the rings was very greatly increased by tarnishing the surface, for which milk much diluted with water is very convenient. It is advantageous to form a diverging beam by transmitting a beam of sunlight through a pretty largo lens, at the focus of which is to be placed the small hole of the screen. In this way of operating the experiment is one of remarkable beauty. On slightly inclining the mirror, the phenomenon changes in a very remarkable way, a set of coloured rings continually opening out from a point midway between the luminous point, or image of the sun in the focus of the Ions, and its image formed by regular reflection. The experiment may be varied in a very beautiful way by dispensing with screen and sunlight, and simply placing a small taper-flame in front of the tarnished mirror, in such a position as to coincide with its inverted image. On viewing the coincident flame and image, they are seen surrounded by a splendid series of coloured rings, which appear to have a determinate position in the air like an actual object.
These phenomena are known as the colours of thick plates. They have been shown to arise from the interference of two streams of light, whereof one is scattered on entering the glass, and then regu larly reflected at the back and refracted out ; and the other enters the glass by regular refraction, and after regular reflection at the back is scattered in emerging. At a point coinciding with the luminous point and its image, the two scattered streams follow the course of the regular light, and therefore their difference of path is nothing, and the ditferenee of path of the two scattered streams, which reach a point at no great distance from the former, will accordingly be sufficiently mall to allow the streams to manifest the ordinary phenomena of interference. The theory of the rings for the most important case,
that In which the luminous point is in the centre of curvature of the mirror, Is given in Sir John Ilerschel's Treatise on Light,' arta. C79, &c.
Dr. Whewell and M. Quetelet observed a system of coloured bands, which are seen when the flame of a candle held near the eye is viewed by reflection in a common looking-glass, several feet off, with a tar nished surface. To see them distinctly, the imago of the flame must be seen distinctly, so that an eyeglass must he used if required. They change with every change of position of the candle or of time eye. and with both eyes a double system is seen, one with each eye. Their explanation depends on the same principles as that of the rings formed by concave mirrors of quicksilvered glass, and the theory of both kinds will be found treated with great detail in a paper published in the • Cambridge Philosophical Transactions,' vol. ix., part 2, p. 147.
The subject of diffraction has been already briefly considered in a special article. [DIFFRACTION or LICUT1 The full comparison of theory and experiment with reference to this curious and interesting class of phenomena, requires far too much mathematical calculation to be here introduced, and reference must be made to Airy's Tract on the Undulatory Theory, or other works in which the question is treated. Suffice it to remark, that the undulatory theory has led to a most complete explanation of the phenomena, qualitatively and quan titatively, in their minutest details.
There is one phenomenon, the astronomical phenomenon of aberra tion, the explanation of which on the corpuscular theory is so simple as to attract little notice, but which presents a serious difficulty on the theory of undulations. To account for it, it has been supposed by Dr. Young and others, that the earth in its motion round the sun passes through the ether without. disturbing it, allowing it to pass between its own particles like the wind through a grove of trees. Startliog as this hypothesis ia, we ought not to reject it on the strength merely of previous notions respecting such a mysteriously subtle medium as the luminiferous ether, if we were fairly led to it. But we are not obliged to have recourse to it, for the phenomenon admits of explanation by attributing to the ether a motion, due to bodies passing through it, which is of a kind with which we have a great deal to do in hydrodynamics. (See ' Philosophical Magazine,' vol. mil (1845), p. 9, and several subsequent articles.) In the explanation of the phenomena which we have hitherto con sidered, nothing depends upon the direction of vibration of the particles of ether which transmit the waves, but the phenomena of polarisation lead ua to suppose that the vibrations are transverse to the direction of propagation. [POLARISATION OF laenr.) This supposition being admitted, the curious and complicated phenomena of the interference of polarised light are explained with beautiful simplicity. Wo have in this article to consider how far the same hypothesis of transverse vibrations helps us towards an explanation of double refraction.