If we take a horizontal line across the spectrum, we shall see what particular colours are missing from the reflected light which falls on the part of the slit corresponding to that line. The colours of some objects, such as of the opal, and the lovely colouring of some feathers are due to interference of light. The partial scattering of different rays by small particles will also cause light to be coloured, as we shall see in the experiments we shall make to imitate the colour of sunlight at various altitudes of the sun. We may, however, take it as a rule that the colour of objects is produced by the greater or less absorption of some rays, and the reflection in the case of opaque bodies, or the transmission, in the case of transparent bodies, of the remainder. CHAPTER VI. Scattered Light--Sunset Colours--Law of the Scattering by Fine Particles--Sunset Clouds--Luminosities of Sunlight at different Altitudes of the Sun. It is probable that we should be able to ascertain approximately the true colour of sunlight (if we may talk of the colour of white light) if we could collect all the light from a cloudless sky, and condense it on a patch of sunlight thrown on a screen. For skylight is, after all, only a portion of the light of the sun, scattered from small particles in the atmosphere, part of the light being scattered into space, and part to our earth. The small particles of water and dust--and when we say small we mean small when measured on the same scale as we measure the lengths of waves of light--differentiate between waves of different lengths, and scatter the blue rays more than the green, and the green than the red; consequently what the sun lacks in blue and green is to be found in the light of the sky. The effect that small water particles have upon light passing through them can be very well seen in the streets of London at night, when the atmosphere is at all foggy. Gaslights at the far end of a street appear to become ruby red and dim, and half-way down only orange, but brighter, whilst close to they are of the ordinary yellow colour, and of normal brightness. When no fog is present the gas-lights in the distance and close to are of the same colour and brightness, showing that their change in appearance is simply due to the misty atmosphere intervening between them and the observer. We can imitate the light from the sun, after its passage through various thicknesses of atmosphere, in a very perfect manner in the lecture-room, using the electric light as a source. A condensing lens is put in front of the lamp, and in front of that a circular aperture in a plate. Beyond that again is a lens which throws an enlarged image of the aperture on the screen, which we may call our mock sun. If we place a trough of glass, in which is a dilute solution of hyposulphite of soda, carefully filtered from motes as far as possible, in front of the aperture, we have an image of the aperture unaffected by the insertion of the solution.