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to say telescope, but camera, for most of the telescopes astronomers use nowadays are just photographic cameras on a large scale. So we might say that we will journey by camera. But in its early history the telescope was not a camera, and so we will for the present use the name telescope.

The earliest telescopes of any importance were very long and thin. Perhaps you will hardly recognize that object in Fig. 22 as a telescope; one end is high up in the air and the other is down on the ground. Sometimes these telescopes were 200 feet long, and they must have been extraordinarily difficult to manage. The astronomers who used them, and really got very accurate results with them, must have been men of great skill. Why did they make them so terribly long when we in modern days have been satisfied with much shorter telescopes? The reason was that they were bothered by colour. We shall see how colour comes into the question in a few moments when we do some experiments; and we shall also see how colour is now a positive advantage to astronomers, when properly used; but in those early days it was only recognized as a disadvantage which drove astronomers to make their telescopes so long and thin that they became difficult to manage. In Fig. 23 is a way of making a long telescope, suggested by the great astronomer Hevelius; he was very proud of his invention and wrote a book about it; and the whole invention was that you need not have an actual tube for the telescope (made of four planks, like a long thin box; that was the way some of them were made), but that a single plank would suffice, if only a lot of circular diaphragms were attached to