Conversion of Relief by Inverted Vision. 435 



When, as in fig. -t, the concavity is seen as a convexity, let 

 it be viewed more and more obliquely. The elliptical margin 

 of the convexity mil always be visible, which is impossible in a 

 real convexity; and the elevated apex will gradually sink till 

 the elliptical margin becomes a straight line, and the imaginary 

 convexity completely leveled. The struggle between truth and 

 error is here so singular, that while one part of the figure 

 m A n has become concave, the other part retains its con- 

 vexity ! 



In like manner, when a convexity is seen as a concavity, 

 the concavity loses its true shape, as it is viewed more and 

 more obliquely, till its remote elliptical margin is encroached 

 upon by the apex of the convexity ; and, towards an inclina- 

 tion of 90°, the concavity disappears altogether, under cir- 

 cumstances analogous to those already described. 



If, in place of using an inverting eye-piece, we invert the 

 concavity m Aw, by looking at its image in the focus of aeon- 

 vex lens, it will sometimes appear a convexity, and sometimes 

 not. In this form of the experiment the image of the con- 

 cavity, and consequently its apparent depth, is greatly dimi- 

 nished. Hence any trivial cause, such as a preconception of 

 the mind, or an approximation to a shadow, or a touch of the 

 hollow by the point of the finger, will either produce a con- 

 version, or prevent it. 



In the preceding experiments we have supposed the con- 

 vexity to be high and the concavity deep and circular, and 

 we have supposed them also to be shadowless, or illuminated 

 by a qiiaquaversus hght, such as that of the sky in the open 

 fields. This was done to get rid of all secondary causes, which 

 interfere with and modify the normal cause when the concavi- 

 ties and convexities are shallow, and have distinct shadows, 

 or when the concavity has the shape of an animal, or any 

 body which we are accustomed to see convex. 



Let us now suppose that a strong shadow is thrown upon 

 the concavity. In this case the normal experiment, already 

 explained and shown in fig. 5, is much more perfect and satis- 

 factory. The illusion is complete, and invariable when the 

 concavity is in an extended surface ; and it as invariably dis- 

 appears when it is in a narrow stripe. 



In the secondary forms of the experiment, the inversion of 

 the shadow becomes the principal cause of the illusion ; but, 

 in order that the result may be invariable, or nearly so, the 

 concavities must be shallow, and the convexities a little raised. 

 At great obli(juities, however, this cause of the conversion of 

 form ceases to produce the illusion, and in varying the incli- 

 nation from 0'' to 90", the cessation takes place sooner with 

 2 G 2 



