100 LETTERS ON NATURAL MAGIC. 



it is surrounded. In like manner the insensible spot on 

 the retina is stimulated by a borrowed light, and the 

 apparent defect is so completely removed that its exist- 

 ence can be determined only by the experiment already 

 described. 



Of the same character, but far more general in its effects, 

 and important in its consequences, is another illusion of 

 the eye which presented itself to me several years ago. 

 When the eye is steadily occupied in viewing any parti- 

 cular object, or when it takes a fixed direction while the 

 mind is occupied with any engrossing topic of speculation 

 or of grief, it suddenly loses sight of, or becomes blind to, 

 objects seen indirectly, or upon which it is not fully 

 directed. This takes place whether we use one or both 

 eyes, and the object which disappears will reappear with- 

 out any change in the position of the eye, while other 

 objects will vanish and revive in succession without any 

 apparent cause. If a sportsman, for example, is watching 

 with intense interest the motions of one of his dogs, his 

 companion, though seen with perfect clearness by indirect 

 vision, will vanish, and the light of the heath or of the 

 sky will close in upon the spot which he occupied. 



In order to witness this illusion, put a little bit of 

 white paper on a green cloth, and within three or four 

 inches of it, place a narrow strip of white paper. At the 

 distance of twelve or eighteen inches, fix one eye steadily 

 upon the little bit of white paper, and in a short time a 

 part or even the whole of the strip of paper will vanish as 

 if it had been removed from the green cloth. It will 

 again reappear, and again vanish, the effect depending 

 greatly on the steadiness with which the eye is kept fixed. 

 This illusion takes place when both the eyes are open, 

 though it is easier to observe it when one of them is 

 closed. The same thing happens when the object is 

 luminous. When a candle is thus seen by indirect vision, 



