EFFECT OF RETARDATION.
Now, all that we have said regarding the retardation of the waves of water, by a whole undulation and a semi-undulation, is perfectly applicable to the case of light. Two luminous points may be placed near to each other so as to resemble the two stones dropped into the water; and when the light of these is properly received upon a screen, or directly upon the retina, we find that at some places the action of the rays upon each other produces darkness, and at others augmented light. The former places are those where the rays emitted from one point are an odd number of semi-undulations in advance of the rays sent from the other; the latter places are those where the difference of path described by the rays is either nothing, or an even number of semi-undulations. Supposing a and b ([Fig. 17]) to be two such sources of light, and s r a screen on which the light falls; at a point l, equally distant from a and b, we have light; at a point d, where a d is half an undulation longer than b d, we have darkness; at l', where a l' is a whole wave-length, or two semi-undulations, longer than b l', we again have light; and at a point d', where the difference is three semi-undulations, we have darkness; and thus we obtain a series of bright and dark spaces as we recede laterally from the central point l.
Let a bit of tin foil be closely pasted upon a piece of glass, and the edge of a penknife drawn across the foil so as to produce a slit. Looking through this slit at a small and distant light, we find the light spread out in a direction at right angles to the slit, and if the light looked at be monochromatic, that is, composed of a single colour, we shall have a series of bright and dark bars corresponding to the points at which the rays from the different points of the slit alternately coincide and interfere upon the retina. By properly drawing a knife across a sheet of letter-paper a suitable slit may also be obtained; and those practised in such things can obtain the effect by looking through their fingers or their eyelashes.
CHROMATIC EFFECTS.
But if the light looked at be white, the light of a candle for example, or of a jet of gas, instead of having a series of bright and dark bars, we have the bars coloured. And see how beautifully this harmonizes with what has been already said regarding the different lengths of the waves which produce different colours. Looking again at [Fig. 17] we see that a certain obliquity is necessary to cause one ray to be a whole undulation in advance of the other at the point l'; but it is perfectly manifest that the obliquity must depend upon the length of the undulation; a long undulation would require a greater obliquity than a short one; red light, for example, requires a greater obliquity than blue light; so that if the point l' represents the place where the first bar of red light would be at its maximum strength, the maximum for blue would lie a little to the left of l'; the different colours are in this way separated from each other, and exhibit themselves as distinct fringes when a distant source of white light is regarded through a narrow slit.
By varying the shape of the aperture we alter the form of the chromatic image. A circular aperture, for example, placed in front of a telescope through which a point of white light is regarded, is seen surrounded by a concentric system of coloured rings. If we multiply our slits or apertures the phenomena augment in complexity and splendour. To give some notion of this I have copied from the excellent work of M. Schwerd the annexed figure ([Fig. 18]) which represents the gorgeous effect observed when a distant point of light is looked at through two gratings with slits of different widths.[B] A bird's feather represents a peculiar system of slits, and the effect observed on properly looking through it is extremely interesting.
COLOURS OF THIN FILMS.
There are many ways by which the retardation necessary to the production of interference is effected. The splendid colours of a soap-bubble are entirely due to interference; the beam falling upon the transparent film is partially reflected at its outer surface, but a portion of it enters the film and is reflected at its inner surface. The latter portion having crossed the film and returned, is retarded, in comparison with the former, and, if the film be of suitable thickness, these two beams will clash and extinguish each other, while another thickness will cause the beams to coincide and illuminate the film with a light of greater intensity. From what has been said it must be manifest that to make two red beams thus coincide a thicker film would be required than would be necessary for two blue or green beams; thus, when the thickness of the bubble is suitable for the development of red, it is not suitable for the development of green, blue, &c.; the consequence is that we have different colours at different parts of the bubble. Owing to its compactness and to its being shaded by a covering of débris from the direct heat of the sun, the ice underneath the moraines of glaciers appears sometimes of a pitchy blackness. While cutting such ice with my axe I have often been surprised and delighted by sudden flashes of coloured light which broke like fire from the mass. These flashes were due to internal rupture, by which fissures were produced as thin as the film of a soap-bubble; the colours being due to the interference of the light reflected from the opposite sides of the fissures.