CONVERSION OF CAMEOS INTO INTAGLIOS. 177 



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 light 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 seat 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 careless observers cannot but know that the shadow 



Fig. 17. 



of a hollow is always on the side next the light, and the 

 shadow of a protuberance on the side opposite to the 

 light ; but if the object is the raised impression of a seal 

 upon wax, I have found that when inverted it still seemed 

 raised to the three youngest of six persons, while the 

 three eldest were subject to the deception. 



This illusion may be dissipated by a process of reason- 

 ing arising from the introduction of a new circumstance 

 in the experiment. Thus, let R L, Fig. 18, be the in- 

 verted seal, which consequently appears raised, and let 

 an opaque and unpolished pin A be placed on one side of 

 the seal. Its shadow will be of course opposite the 

 candle as at B. In this case the seal which had become 



