This hand stroboscope is a very simple way that the principle of stroboscopy is taught. The eye looks through a narrow slit while a disc with 12 slots is spun by the user's finger as a set of slots go past the original stationary slot. This acts as a shutter and the eye sees rapid on/off images of a moving object. It the speed of the disc is correct the image will appear to be stationary or slowly moving forwards or backwards. This model has 2 discs which can be easily adjusted to provide either 1, 2, 3, 4, 6, or 12 slots in the rotating disc. Instructions included. Made in Australia