108 Mr Magnus's Analysis of Picrosmine. 



singular illusion we have seen on several days in July. The 

 telescope had no effect whatever in producing it, as it showed 

 objects erect. 



An illusion of an analogous nature we once observed when 

 looking at the Abbey Church of Paisley, where the clustered 

 columns of a Gothic pillar all sunk into hollow flutings. The 

 cause of this deception was not discovered, but it must have 

 arisen from some mistaken notion respecting the direction in 

 which the object was illuminated. 



The last species of illusion of this nature, and perhaps the 

 most remarkable of all of them, may be produced by a con- 

 tinued effort of the mind to deceive itself. If we take one 

 of the intaglio moulds used for making the bas-reliefs of that 

 able artist Mr Henning, and direct the eye to it steadily with- 

 out noticing surrounding objects, we may coax ourselves into 

 the belief that the intaglio is actually a bas-relief. It is diffi- 

 cult at first to produce the deception, but a little practice never 

 fails to accomplish it. 



We have succeeded in carrying this deception so far, as 

 to be able, by the eye alone, to raise a complete hollow 

 mask of the human face into a projecting head. In order 

 to do this, we must exclude the vision of other objects ; 

 and also the margin or thickness of the cast. This experi- 

 ment cannot fail to produce a very great degree of surprise 

 in those who succeed in it ; and it will no doubt be re- 

 garded by the sculptor who can use it as a great auxiliary 



in his art. 



D. B. 



Akt. XX. — Analysis of Picrosmine. By Gustavus Mag- 

 nus, Esq. of Berlin. Communicated by the Author. 



A description of this mineral, which was first ascertained 

 by Mr Haidinger to form a distinct species, is contained in 

 the Treatise on Mineralogy by Professor Mohs,* and in 

 a former Number of this Journal.-f The description was 



* Grundruss der Mineralogie, Th. ii. English Translation, vol. iii. 

 p. 137. 



t Sec vol. ii. p. 376, April 1825. 



