436 071 the Conversion of Relief by Inverted Vision. 



deep than with shallow cavities. The reason of this is, that 

 the shadow of a concavity is very different at great obliquities 

 from the shadow of a similar convexity. The shadow never 

 can emerge out of a cavity so as to darken the surface in which 

 the cavity is made ; whereas the shadow of a convexity soon 

 extends beyond the outline of its base, and finally throws a 

 long stripe of darkness over the surface on which it rests. 

 Hence it is impossible to mistake a convexity for a concavity, 

 whenever its shadow extends beyond its base. 



When the concavity is a horse or a dog upon a seal, it 

 will often rise into a convexity when seen through a single 

 lens which does not invert itj but the illusion disappears at 

 great obliquities. In this case the illusion is favoured, or 

 produced, by two causes': the first is, that the convex form of 

 the horse or dog is the one which the mind is most disposed 

 to seize; and the second is, that we use only one eye, with 

 which we cannot measure depths as well as with two. The 

 illusion, however, still takes place when we employ a lens 

 three or more inches wide, so as to admit the use of both eyes, 

 but it is less certain, as the binocular vision enables us to keep 

 in check, to a certain degree, the other causes of illusion. 



The influence of these secondary causes is strikingly dis- 

 played in the following experiment. In the armorial bearings 

 upon a seal, the shield is often more deeply cut than the sur- 

 rounding parts. With binocular 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 tl;e 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 illumi- 

 nation, or particular reflexions 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 mothcr-of-jycarl^ 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 portions are illuminated by refrac- 

 tion, at those parts of their circumference where they would 

 have been illuminated had they been convexities*. But the 



• In examining, under the microscope, the shallow fluid cavities within 



