142 SCIENTIFIC RECREATIONS. 



illuminated by a flash of lightning, because the eye has not time to take in 

 the motion in the instant of time, for the spokes of the wheel are not moving 

 fast enough to convey the impression of motion in that half second to the 

 eye ; yet the perfect outline of the wheel is distinctly visible. 



Indeed, distinct vision can be exercised in a very small fraction of a 

 second. It was calculated by Professor Rood, and proved by experiment, 

 that forty billionths of a second is sufficient time for the eye to distinguish 

 letters on a printed page. It this instance the illuminating power was an 

 electric spark from a Ley den Jar. 



We have remarked upon the distinctness with which we can see an object 

 when we direct our gaze upon it, and this appears a self-evident proposition ; 

 but have any of our readers remarked the curious fact that when they want 

 to see a faint and particular star in the sky it will at once disappear when 

 they gaze at it ? The best way to see such very faint orbs as this is to look 

 away from them, a little to one side or the other, and then the tiny point 

 will become visible again to the eye. There is also a degree of phospho- 

 rescence in the eye, which any one who receives a blow upon that organ will 

 readily admit. Even a simple pressure on the closed lid will show us a circle 

 of light and " colours like a peacock's tail," as the great Newton expressed 

 it. There are many occasions in which light is perceived in the eye generally 

 the result of muscular action ; and the Irish term to " knock fire out of my 

 eye " is founded upon philosophical facts. 



We are many of us aware of " spots" on our eyes when our digestion 

 is out of order, and the inability of the eye to see figures distinctly in a 

 faint light within a proper seeing distance, too has often given rise to the 

 " ghost." These shadowy forms are nothing more than affections of the eye, 

 and, as well remarked in Brewster's Letters on Natural Magic, "are always 

 white because no other colour can be seen." The light is not sufficiently 

 strong to enable the person to see distinctly ; and as the eye passes from side 

 to side, and strives to take in the figure, it naturally seems shadowy and 

 indistinct, and appears to move as our eyes move. " When the eye dimly 

 descries an inanimate object whose different parts reflect different degrees 

 of light, its brighter parts may enable the spectator to keep up a continued 

 view of it ; but the disappearance and reappearance of its fainter parts, and 

 the change of shape which ensues, will necessarily give it the semblance of 

 a living form ; and if it occupies a position which is unapproachable, and 

 where animate objects cannot find their way, the mind will soon transfer to 

 it a supernatural existence. In like manner a human figure shadowed forth 

 in a feeble twilight may undergo similar changes, and after being distinctly 

 seen while it is in a situation favourable for receiving and reflecting light, 

 it may suddenly disappear in a position before and within the reach of the 

 observer's eye ; and if this evanescence takes place in a path or roadway 

 where there is no sideway by which the figure could escape, it is not easy 

 for an ordinary mind to efface the impression which it cannot fail to receive." 

 This will account for many so-called "ghosts." 



