put this prism in the path of a straight beam of light it is bent aside so that it no longer falls on the screen in front, but on this other screen to the side; and at the same time it becomes beautifully coloured (Fig. 26). The fact is that blue colours are more bendable than yellow, and yellow than red;

Fig 27. so that though all the colours start together in the same ray out of the lantern, when the prism bends them, some bend more than others and we get them separated. Now those colours have become lately of the greatest value to astronomers, but at first they were merely a nuisance; they interfered with the use of lenses in telescopes. A lens is a piece of glass with curved surfaces; when rays of light strike those surfaces they are bent to a focus (Fig. 27), as is wanted for making a telescope. But the bending introduces colour which is not wanted; and yet they could not in early days get rid of the colours without also getting rid of the bending to focus. All they could do was to keep the colour effects as small as possible by making the bending also small, which means that the rays have to travel a long way before they come to focus. That is why the early telescopes made with lenses were so long and thin; if they had been shorter and wider the colour effects would have been so great that the observations would have been use-