"Thou canst not wave thy staff in the air, Or dip thy paddle in the lake, But it carves the brow of beauty there, And the ripples in rhymes the oar forsake."
[Illustration: Fig. 16. Chasing produced by waves.]
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_.
[Illustration: Fig. 17. Diagram explanatory of Interference.]
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.