104 LETTEKS ON NATUKAL MAGIC. 



Fig. 18. 



explain to us why on some occasions an acute 

 observer will elude the deception, while every 

 other person is subject to it. Let us suppose 

 that a particle of dust, or a little bit of wax, 

 capable of giving a shadow, is adhering to the 

 surface of the seal, an ordinary observer will 

 take no notice of this, or if he does, he will pro- 

 bably not make it a subject of consideration, and 

 will therefore see the head on the seal raised into 

 a cameo ; but the attentive observer, noticing the 

 little protuberance, and observing that its shadow 

 lies to the left of it, will instantly infer that the 

 light comes in that direction, and will still see 

 the seal hollow. 



I have already mentioned that in some cases 

 even the sense of touch does not correct the 

 erroneous perception. We of course feel that 

 the part of the hollow on which the finger is 

 placed is actually hollow ; but if we look at the 

 other part of the hollow it will still appear raised. 



By using two candles yielding different degrees 

 of light, and thus giving an uncertainty to the 

 direction of the light, we may weaken the illusion 

 in any degree we choose, so as to overpower it 

 by touch, or by a process of reasoning. 



I have had occasion to observe a series of ana- 

 logous phenomena arising from the same cause, 



