(9) The next time that you listen to an orchestra, pick out the tones of the various instruments, and try to describe their timbre; do not be afraid to string adjectives together, but be sure that you hear what you put down. Later, look up in a reference-book the composition of these various compound tones, and see if there is any correlation between your description and the number and loudness of the overtones. (10) If you drop a block of wood on a desk, the sound is simply noisy. If the same block forms part of a xylophone scale, and is struck with the wooden hammer, it gives a tone. How is this? (11) When you next go to a reception, stand outside the main rooms for a minute, and try to determine the pitch of the buzz of voices; try to sing the pitch yourself. Is the buzz tonal or merely noisy? (12) When you are listening to beats, do you hear one beating tone, or both the primary tones beating? If one tone only, is it identical with either of the primaries? (13) Test the law of visual antagonism by getting the after-images of a number of colours. (14) To prove normal colour-blindness, get a small square of red glass; stand before a window, with your left eye closed and your right eye fixed upon some distant point; bring the red glass slowly into the field, with the left hand, and note its changes. (15) Can you suggest experiments for working out in detail the laws of visual contrast? Try to think what sort of things would be likely to enhance or to reduce the contrast-effect. (16) Could a man go through life, and take an ordinary place in society, without knowing that he was colour-blind? Give your reasons. (17) Blue and yellow are antagonistic; yet blue and yellow paints, mixed on the palette, give green. How is this? (18) Dalton says: “In lecturing on optics I got six ribands,—blue, pink, lilac,—and red, green, and brown,—which matched very well, and told the curious audience so. One gentleman came up immediately afterwards and told me he perfectly agreed with me; he had not remarked the difference by candlelight.” How could these triads have been confused? and would the candlelight make any difference? References