Achromatism of Lenses, a device usually obtained by the use of compound lenses, to remedy the effects of chromatic aberration. A combination of lenses made of different kinds of glass, and of definite focal lengths, will enable us to unite as many coloured images as there are lenses. For ordinary purposes it is only necessary to combine two of the more intense colours, such as the orange-yellow and the green-blue. This is done with crown and flint-glass lenses. It has been discovered recently that single lenses, if manufactured of carefully-prepared glass, may be made achromatic. Blair achieved the same result a century ago by the use of fluid lenses.