In experiments on the spectrum, it is usual to employ a slit through which the light is admitted, a prism to analyse the light, and one or more lenses to bring the rays of each distinct kind to a distinct focus on the screen. The most perfect arrangement is that adopted by M. Kirchhoff, in which two achromatic lenses are used, one before and the other after the passage of the light through the prism, so that every pencil consists of parallel rays while passing through the prism.

But when the observer has not achromatic lenses at his command, or when, as in the case of the highly refrangible rays, or the rays of heat, he is restricted in the use of materials, it may still be useful to be able to place the lenses and prism in such a way as to bring the rays of all colours to their foci at approximately the same distance from the prism.