The use of the `fly's eye' apparatus to study crystal structures containing atoms of different scattering powers

The `Fly's Eye' apparatus (Bragg, 1944), as improved by Stokes (1946), consists of a regular array of tiny perspex lenses embossed on a perspex sheet. Its purpose is to form a multiple photograph of a proposed crystal structure projected along some crystallographic direction. This multiple photograph can then be used as a diffraction grating for visible light, and it will give orders of diffraction which have intensities similar to the X-ray reflexions from the real crystal in a zone corresponding to the direction of projection.