less. Newton and Gregory and others therefore turned aside from the making of lenses and proposed to use mirrors, which can also collect rays of light to a focus if they are hollowed out to a particular shape. You can try this for yourselves with the reflector of a common oil-lamp; it will bring rays to a focus just as a burning glass will (Fig. 28).
Fig 28.
But now let me show you another prism. When this is put in the path of the rays you see they are not broken back at all; they still pass straight to the screen; but at the same time they are wonderfully coloured. How is this? We learnt that colour was a result of refraction, and yet here is colour without refraction. The fact is, this prism is really made up of two; one bends the rays aside and colours them, the other bends them back again without undoing the colour entirely. That is a new and valuable idea, that one bending can balance another and yet the colour effects do not balance, because it suggests to us at once that we ought to be able to do just the opposite, that is to balance the colour effects and leave the bendings unbalanced; and in this way we can make a lens which will not show colour. When this was once realized astronomers went back to lenses for their large telescopes, and they have suc-
