Many double stars show most beautifully contrasted colours: among them are pairs of yellow and rose-red, golden and azure, orange and purple, orange and lilac, copper-colour and blue, apple-green and cherry-red, and so on. In the Southern Hemisphere there is a cluster containing so many stars of brilliant colours that Sir John Herschel named it 'the Jewelled Cluster.' I expect most of you have seen an advertisement of Pear's Soap, in which you are asked to stare at some red letters, and then look away to some white surface, such as a ceiling, when you will see the same letters in green. This is because green is the complementary or contrasting colour to red, and the same thing is the case with blue and yellow. When any one colour of either of these pairs is seen, it tends to make the other appear by reaction, and if the eye gazed hard at blue instead of red, it would next see yellow, and not green. Now, many people to whom this curious fact is known argue that perhaps the colours of the double stars are not real, but the effect of contrast only; for instance, they say a red star near a companion white one would tend to make the companion appear green, and so, of course, it would. But this does not account for the star colours, which are really inherent in the stars themselves, as may be proved by cutting off the light of one star, and looking only at the other, when its colour still appears unchanged. Another argument equally strong against the contrast theory is that the colours of stars in pairs are by no means always those which would appear if the effect was only due to complementary colours. It is not always blue and yellow or red and green pairs that we see, though these are frequent, but many others of various kinds, such as copper and blue, and ruddy and blue.