THUNDER-STORMS. 513 



indistinct, until, at length, a velocity will be attained which will cause a spoke 

 to be continually seen at the opening o, in the same manner as if the whn-1 

 were at rest, and the spoke a were placed behind the aperture. Now, since 

 it is certain that in this case the presence of the spokes at the aperture is suc- 

 cessive, and that the intervals which the spokes are absent bear to the intervals 

 of their presence, the proportion of the breadth of the spokes to the breadth 

 of the spaces between them, it necessarily follows that the eye perceives a 

 spoke at the aperture during the intervals when no spoke is present there. 



This circumstance is accounted for by considering the manner in which vis- 

 ion is effected by means of the mechanism of the eye. The light proceeding 

 from a visible objeqt, entering the pupil, strikes the retina and produces in it a 

 certain vibration, which vibration is the immediate cause of the perception of 

 the object from which the light has been transmitted. After the object has 

 ceased to transmit light to the eye, this vibration continues for a certain time, 

 just as the vibration of a musical string continues for a certain interval after 

 the bow which put it into vibration has been withdrawn ; and, as the vibration 

 of the string continued, after the bow is withdrawn, produces the perception of 

 a proportionately prolonged sound, so the vibration of the retina, after the visi- 

 ble object has been withdrawn, produces a proportionately prolonged perception 

 of its presence. In fact, there is no damper in the mechanism of the eye to 

 stop the effect of the action of light at the instant that action ceases. It is, 

 therefore, an interesting physiological problem to determine how long after that 

 visible object is withdrawn, and the action of light ceases, the effect on the 

 retina remains, and the object continues to be seen. This problem is beauti- 

 fully solved by the apparatus above described. The velocity of the wheel be- 

 ing gradually augmented until the spoke appears to be continually present at 

 the opening, it has been found that t his effect is produced when the wheel performs 

 one complete revolution in a second of time. Since the space round the centre 

 of the wheel is equally divided by the ten spokes, it follows that in this case 

 the interval between the arrival of two successive spokes at the opening is one 

 tenth of a second, and this must, therefore, be the duration of the impression 

 of an object on the retina after it has been withdrawn. If the duration were 

 less than this the colored spoke would not appear continually at the aperture o 

 when the wheel revolves in one second, but would alternately appear and dis- 

 appear. If it were greater, a less velocity than one revolution per second 

 would be sufficient to cause its continuous appearance. 



Since there is nothing in what has been stated to render it necessary that 

 the aperture, through which the spokes are seen, should be in the vertical, 

 rather than any other position, it follows that in whatever position, round the 

 centre, that aperture be placed, a spoke will appear to be continually behind 

 it, so long as the wheel revolves at a rate of not less than one revolution per 

 second. 



If, therefore, there be two or more such apertures made in the screen, a 

 spoke will appear constantly behind each of them. In fine, if there be an in- 

 finite number of such apertures round the centre, or, in other words, il the 

 screen be altogether removed, spokes will be seen in every direction round the 

 centre without any open spaces between them, or what is the same, the wheel 

 will appear as a circular disk of uniform red, no spokes being distinguishable. 



We have here supposed that the wheel is continually illuminated. It is ne- 

 cessary now to inquire how long light must shine upon it in order that, revolv- 

 ing once per second, it may appear as a plane disk without spaces between the 

 spokes. If the light fall upon it only for an instant, that is, an infinitely short 

 time, then the wheel will be distinctly seen, for the tenth of a second, in the 

 position which it had when the light fell upon it. The spokes will be as distinct- 



33 



