CHAPTER XII. 



OPTICAL ILLUSIONS CONTINUED EXPERIMENTS THE TALKING HEAD- 

 GHOST ILLUSIONS. 



THE enumeration of optical illusions is so considerable that we have no 

 intention of describing them all, and will merely cite a few other examples 

 The following facts have been communicated to us by M. Nachet : 



When examining algae under the microscope, we notice the spaces 

 which separate the streaks ornamenting the silicious covering of these 

 various organisms, and it is explained that they are formed by hexagons 



Fig. 132. --Hexagonal appearance formed by circles joined together. 



visible only when we examine the object with a powerful microscope. "For 

 a long time," says M. Nachet, " I occupied myself with the examination of 

 the hexagonal appearance of the points constituting the streaks. Why should 

 these hexagons show themselves, and how could they be other than the 

 visible base of small pyramids piled very closely one on the other ; and if 

 this were the case, why were not the points of the little pyramids visible ? 

 Or, was the structure before me analogous to that of the eyes of insects ? 

 Then the carapace would be but a surface of perforated polygonal openings. 

 This latter hypothesis was attractive enough, and would have explained 

 many things ; but some careful observations with very powerful object - 



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