The phenakistiscope may be described (figs. 3 and 4) as consisting of an iron pin _a b_ turning easily on its axis, and passing through two holes in a brass rod _t g_, bent twice at right angles. Attached to one end of the pin is a disc of cardboard, divided into several equal sectors, and pierced near its circumference with as many similar sized rectangular holes (fig. 4). In each sector the same scene is represented, with this difference, that the movements of the objects are so arranged as to be progressive from one extreme to the other. The disc being fastened to the pin _a b_ (fig. 3) by the screw _v_, with the figures facing outwards towards _a_, the whole apparatus is held before a looking-glass by the handle _m_. If the disc be now rotated by the button _b_, and the eye placed opposite one of the square holes in the card, the figures on the disc will appear to move more or less quickly according to the rate at which it is rotated. The three bricklayers in fig. 4 will be seen to pass their bricks from one to the other with perfect regularity if the drawing has been made carefully. Numberless other designs may be made for this little instrument, such as a windmill in full sail, a man working a pump, a conjurer swallowing knives—in fact, any scene with objects in motion may be drawn, and will cause infinite amusement for the long winter evenings. The time during which the impression of any object remains upon the retina appears to be in direct proportion to its brilliancy. For a burning coal it is stated to be about the tenth of a second; consequently, if the stick mentioned at the beginning of the chapter is rotated ten times in a second, a continuous luminous 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. [Illustration: FIG. 4.—Disc of the Phenakistiscope. ]