178 LETTERS ON NATURAL MAGIC. 



a cameo by its inversion, will now sink down into a 

 cavity by the introduction of the pin and its shadow ; for as 

 the pin and its shadow are inverted, as shown in Fig. 18, 

 while the candle retains its place, the shadow of the pin 

 falling in the direction A B is a stronger proof to the 

 eye that the light is coming from the right hand, than 

 the actual knowledge of the candle being on the left 

 hand, and therefore the cameo necessarily sinks into a 

 cavity, or the shadow is now on the same side as the light. 

 This experiment will 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 



Fig. 18. 



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 probably not make it a subject of considera- 

 tion, 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, 



