071 Magnetism, Cohesiofi, Adhesioti and Viscosity. 437 



most interesting cases of conversion of form are those in which 

 the mind alone operates, and receives no aid either from in- 

 version, shadow, or monocular vision. " If we take, as I have 

 elsewhere remarked, one of the IntagHo moulds, used in 

 making the bas-reliefs of that able artist Mr. Henning, and 

 direct the e}es to it steadily, without noticing surrounding 

 objects, we may coax ourselves into the belief that the Intaglio 

 is actually a bas-relief. It is difficult 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 experiment cannot fail to produce 

 a very great degree of surprise in those who succeed in itj 

 and it will, no doubt, be regarded by the sculptor (who can 

 use it) as a great auxiliary in his art*." 



From these observations it will be seen that the conversion 

 of form, excepting in the normal case, depends upon various 

 causes which are effective only under particular conditions ; 

 such as the depth of the hollow or the elevation of the relief, 

 the distance of the object, the sharpness of vision, the use of 

 one or both eyes, the inversion of the shadow, the nature of 

 the object, and the means used by the mind itself to produce 

 the illusion. In the normal case, however, where the cavity 

 or convexity is shadowless, and upon an extended surface, 

 and where inverted vision is used, the conversion of form 

 depends solely on the illusion, which it is impossible to resist, 

 that the side of the cavity or elevation next the eye is actually 

 furthest from it — an illusion not produced by inversion, but 

 by a false judgement respecting the position of the surface on 

 which the form is placed. 



St. Leonard's College, St. Andrews, 

 May 4, 1844. 



LXVIII. Experiments proving the common nature of Mag- 

 netism, Cohesion, Adhesion and Viscosity. By Sir Graves 

 C. Haughton, K.H., M.A., F.R.S., Foreign Associate of 

 the Institute of France, c§-c.t 



Part I. 



TTARIOUS methods of experimenting have been employed 

 ^ of late years to extend the knowledge of magnetic phae- 



the substance of a film of sulphate of lime, described in the Edinburgh 



Transactions, vol. x. p. .35, they frequently, appeared as elevations on the 



surface of the plate next the eve. 



• Edinburgh Journal of Science, No. VIII. p. 101), Jan. 1826. 

 t Communicated by the Author. 



