Ferrous sulphate heptahydrate, polarised light micrograph. Ferrous sulphate is used to treat iron deficiency, which causes anaemia. Anaemia is a condition in which the body lacks sufficient red blood cells by having insufficient iron present in the body. This is an image of a thin crystalline film of ferrous sulphate imaged in polarised light. The colour is due to both the birefringence exhibited by the ferrous sulphate crystals and the waveplate inserted in the optical train of the microscope. Magnification: x70 when printed 10 centimetres wide.