190 



LETTERS ON NATURAL MAGIC. 



covered with black oxide are more luminous than the 

 adjacent parts, on account of the superior radiation of 

 light by the black oxide which covers them. 



By the means now described invisible writing might be 

 conveyed by impressing it upon a metallic surface, and 

 afterwards erasing it by grinding and polishing that 

 surface perfectly smooth. When exposed to a proper 

 degree of heat, the secret would display itself written in 

 oxidated letters. Many amusing experiments might be 

 made upon the same principle. 



A series of curious and sometimes alarming deceptions, 

 arises from the representation of objects in perspective 



direction in reference to the person who views them is 

 entirely a different matter ; and Dr. Wollaston is of 

 opinion, that we are not guided by the eyes alone, but are 

 unconsciously aided by the concurrent position of the 

 entire face. 



If a skilful painter draws a pair of eyes with great 

 correctness directed to the spectator, and deviating from 



