4. _The effect of light upon the eye is not instantaneous, but continues for a short space of time._ This may be proved and illustrated by the following examples:--If a stick--or a ball connected with a string--be whirled round in a circle, and a certain degree of velocity given it, the object will appear to fill the whole circle it describes. If a lighted firebrand be whirled round in the same rapid manner, a complete circle of light will be exhibited. This experiment obviously shows that the impression made on the eye by the light from the ball or the firebrand--when in any given point of the circle--is sufficiently lasting to remain till it has described the whole circle, and again renews its effect, as often as the circular motion is continued. The same is proved by the following considerations:--We are continually shutting our eyes, _or winking_; and, during the time our eyes are shut, on such occasions, we should lose the view of surrounding objects, if the impression of light did not continue a certain time while the eye-lid covers the pupil; but experience proves that during such vibrations of the eye-lids, the light from surrounding objects is not sensibly intercepted. If we look for some time steadily at the light of a candle, and particularly, if we look directly at the sun, without any interposing medium, or if we look for any considerable time at this luminary, through a telescope with a coloured glass interposed--in all these cases, if we shut our eyes immediately after viewing such objects, we shall still perceive a faint image of the object, by the impression which its light has made upon our eyes.