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the plank at intervals. His whole invention was to save three of the four planks and use one only! Perhaps I am not quite fair, for he was also rather proud of his invention of the lifting gear and of the floor under which the telescope could be stowed when out of use.

Fig. 23.

You will readily see the great difficulties of using these machines, so that astronomers cast about for some other way of getting over the difficulty of colour. It occurred to several people independently that if a mirror could be used instead of a lens to bring the light to focus there would then be no colour and therefore no difficulty. We will explain this fact and illustrate it by experiment in a few minutes; but first let me say a word or two about the consequences of this new move in the making of