The cards, which create a persistence of vision effect similar to a thaumatrope, were intended to correct short/long sight in children. The instructions summarised read: 'The preliminary exercises are those with the pictures which show different subjects (1-7) viz: Bird and Cage, Mouse and Trap etc. - These pictures must be used until the squinting eye once at recognises its part of the picture. If it is then considered possible to use both eyes together, the pictures which represent the same picture twice may be taken...The two pictures will soon graduate into one and thus the axis of the eye will find its right position...Common stereoscopes may be used.'