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Rh day, but Sunday's loaf was delivered late—say on Tuesday; if all the others were two days late also then you would still get a loaf every day. But if they got less and less late until Saturday's loaf came as soon as it was baked, then you would get a whole week's loaves in the five days. After that we may suppose the delivery begins to get late again until the next Sunday's loaf is once more two days late—seven loaves in nine days, which is like the low note of a train leaving a station.

If this does not seem clear, never mind: you will perhaps find a better explanation some day. But now I want you to think of light instead of sound, because the stars do not whistle to us, but they do send us light. You remember that we were to compare light and sound several times in this lecture? And I want you to realize that just as there are scales in sound, which you have to practise on the piano, so there are scales in light—nothing more nor less than those beautiful colours which a prism makes for us. We will throw one of those rainbow-coloured strips on the screen; and to fix the likeness of colours to scales in your minds I will put this harmonium underneath the strip and play a scale. The low notes in the bass correspond to the red colour, and as we go up the scale we pass through orange, yellow, etc. all the colours of the rainbow up to violet. You might sing the names to a scale so as to remember this fact.