102 LETTERS ON NATURAL MAGIC. 



cavity or intaglio. If the two candles do not 

 illuminate the seal equally, or if any accidental 

 circumstance produces a belief that the light is 

 wholly or principally on one side, the mind will 

 entertain a corresponding opinion respecting the 

 state of the seal, regarding it as a hollow if it 

 believes the light to come wholly or principally 

 from the right hand, and as a cameo if it believes 

 the light to come from the left hand. 



If we use a small telescope to invert the seal, 

 and if we cover up all the candle but the flame, 

 and arrange the experiment so that the candle 

 may be inverted along with the image, the seal 

 will still retain its concavity, because the shadow 

 is still on the same side with the illuminating 

 body. 



If we make the same experiments with the 

 raised impression of the seal taken upon wax, we 

 shall observe the very same phenomena, the seal 

 being depressed when it alone is inverted, and 

 retaining its convexity when the light is inverted 

 along with it. 



The illusion, therefore, under our consideration 

 is the result of an operation of our own minds, 

 whereby we judge of the forms of bodies by the 

 knowledge we have acquired of Jight and shadow. 

 Hence the illusion depends on the accuracy and 

 extent of our knowledge on this subject ; and 

 while some persons are under its influence, others 

 are entirely insensible to it. When the seal or 

 hollow cavity is not polished, but ground, and 

 the surface round it of uniform colour and 

 smoothness, almost every person, whether young 

 or old, learned or ignorant, will be subject to the 

 illusion; because the youngest and the most 



