CONVERSION OF RELIEF BY INVERTED VISION. 601 



ing experiment. In the armorial bearings upon a seal, the shield is often more 

 deeply cut than the surrounding parts. With hinocular vision the shallow parts 

 rise into a convexity sooner than the shield, or continue so while the shield 

 remains concave ; but if we shut one eye, the shield then becomes convex like the 

 rest. In these experiments with a single lens, a slight variation in the position of 

 the seal, or a slight change in the direction or intensity of the illumination, or par- 

 ticular reflections from the interior of the stone, will favour or oppose the illusion. 

 In viewing the shield, or the deepest portion, with a single lens, a slight rotation 

 of the seal round the wrist, backwards and forwards, will remove the illusion, in 

 consequence of the eye perceiving that the change in the perspective is different 

 from what it should be. 



In a paper in the Edinburgh Journal of Science, already referred to, I have 

 described several other examples of the conversion of form, in which inverted 

 vision is not employed. As seen by the naked eye, hollows in mother of pearl, 

 and other semi-transparent bodies, rise into relief; and the same thing happens 

 on surfaces of agate and woods of various kinds, when transparent circular por- 

 tions are illuminated by refraction, at those parts of their circumference where 

 they would have been illuminated had they been convexities.* But the most 

 interesting cases of conversion of form are those in which the mind alone operates, 

 and receives no aid either from inversion, shadow, or monocular vision. " If we 

 take, as I have elsewhere remarked, one of the Intaglio moulds, used in making 

 the bas-reliefs of that able artist Mr HENNING, and direct the eyes 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 a,t 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 it ; and it will, no doubt, be regarded by the sculptor (who can 

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



From these observations it will be seen that the conversion of Form, except- 

 ing 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 



* In examining, under the microscope, the shallow fluid cavities within 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 eye. 



f Edinburgh Journal of Science, No. VIII. p. 109, Jan. 1826. 



VOL. XV. PART IV. 8 Q 



