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JOURNEYING BY TELESCOPE
101
when you try with a prism you will find the ray behaves as drawn; it gets bent in the same direction both times,
Fig. 25.
and comes out in a line quite turned aside from its original direction AB.
Fig. 26.
But at the same time it becomes coloured, with all the colours of the rainbow. Whenever you get this refraction or "breaking back" you also get colour. When I
