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 

 upon a plane surface. One of the most interesting of 

 these depends on the principles which regulate the 

 apparent direction of the eyes in a portrait. Dr. Wollaston 

 has thought this subject of sufficient importance to be 

 treated at some length in the Philosophical Transactions. 

 When we look at any person we direct to them both our 

 face and our eyes, and in this position the circular iris 

 will be in the middle of the white of the eyeball, or, what 

 is the same thing, there will be the same quantity of white 

 on each side of the iris. If the eyes are now moved to 

 either side, while the head remains fixed, we shall readily 

 judge of the change of their direction by the greater or 

 less quantity of white on each side of the iris. This test, 

 however, accurate as it is, enables us only to estimate the 

 extent to which the eyes deviate in direction from the 

 direction of the face to which they belong. But their 

 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 



