Page:The wonders of optics (1869).djvu/63

Rh ring will appear to be formed. That the time necessary for producing a distinct impression varies with the brilliancy of the object, may be readily guessed from the fact that an electric spark is perfectly visible, although its duration can hardly be measured, while a cannon-ball in flight is only perceptible to the practised eye of the artilleryman, owing to its reflecting only a small quantity of very diffused light.



The second instrument, the thaumatrope, is constructed on the same principle. It consists of a certain number of circular discs of card three or four inches across, which are capable of being turned on their axes