‘With respect to the _duration_ of the impression of light, it has been observed that the teeth of a cog-wheel in a clock were still visible in succession, when the velocity of rotation brought 246 teeth through a given fixed point in a second. In this case it is clear that if the impression made on the eye by the light reflected from any tooth, had lasted without sensible diminution for the 246th part of a second, the teeth would have formed one unbroken line, because a new tooth would have continually arrived in the place of the interior one before its image could have disappeared. If a live coal be whirled round, it is observed that the luminous circle is complete, when the rotation is performed in the (8-1/2)/60th of a second. In this instance we see that the impression was much more durable than the former. Lastly, if an observer sitting in a room direct his sight through a window, to any particular object out of doors, for about half a minute, and then shut his eyes and cover them with his hands, he will still continue to see the window, together with the outline of the terrestrial objects bordering on the sky. This appearance will remain for near a minute, though occasionally vanishing and changing colour in a manner that brevity forbids our minutely describing. From these facts we are authorized to conclude, that all impressions of light on the eye, last a considerable time, that the brightest objects make the most lasting impressions; and that, if the object be very bright, or the eye weak, the impression may remain for a time so strong, as to mix with and confuse the subsequent impressions made by other objects. In the last case the eye is said to be _dazzled_ by the light.’[4] The following experiment has likewise been suggested as a proof of the impression which light makes upon the eye. If a card, on both sides of which a figure is drawn, for example, a bird and a cage, be made to revolve rapidly on the straight line which divides it symmetrically, the eye will perceive both figures at the same time, provided they return successively to the same place. M. D’Arcy found by various experiments, that, in general, the impression which light produces on the eye, lasts about _the eighth of a second_. M. Plateau, of Brussels, found that the impression of different colours lasted the following periods; the numbers here stated being the decimal parts of a _second_. _Flame_, 0.242. or nearly one fourth of a second; _Burning coal_, 0.229; _White_, 0.182, or, a little more than one sixth of a second; _Blue_, 0.186; _Yellow_, 0.173; _Red_, 0.184.