CONVERSION OF RELIEF BY INVERTED VISION. 659 



pear a concavity in Fig. 4, solely because it seems to rise out of the surface 

 M N, which looks upward, as if it had not been inverted by the eye-piece. 



Now, in this experiment, the conversion of the concavity into a convexity 

 depends on two separate illusions, one of which springs from the other. The 

 first illusion is the belief that the surface Fig. 5. 



M N is looking upwards, whereas it is really 

 inverted, as shewn in Fig. 5 ; and the second 

 illusion, which arises from the first, is, that 

 the point n appears farthest from the eye, 

 whereas it is nearest to it, as shewn in Fig. 5. 

 All these observations are equally applicable 

 mutatis mutandis to the vision of convexities ; and hence it follows, that the 

 conversion of relief, occasioned by the use of an inverting eye-piece, is not pro- 

 duced directly by the inversion, but by an illusion, in virtue of which we conceive 

 the remotest side of the convexity or concavity to be nearest our eye when it is 

 not. 



In order to demonstrate the correctness of this explanation, let the concavity 

 m A n be made in a narrow stripe of wood, as in Fig. 5, and let it be viewed, as 

 formerly, through the inverting eye-piece. It will now appear, as in Fig. 5, really 

 inverted, and free from both the illusions which formerly took place. The narrow 

 surface M N being now wholly included in the field of view, and the thickness 

 NO of the stripe of wood distinctly seen, the inversion of the surface MN, which 

 now looks downward, will be at once recognised. The edge n of the concavity 

 will appear nearest the eye,* as it really is, and the concavity, though inverted, will 

 still appear a concavity. The very same reasoning is applicable to a convexity on 

 a narrow stripe of wood. 



When, as in Fig. 4, the concavity is seen as a convexity, let it be viewed 

 more and more obliquely. The elliptical margin of the convexity mill 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 levelled. The struggle between truth and error is here so sin- 

 gular, that while one part of the Figure m A n has become concave, the other part 

 retains its convexity ! 



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 incli- 

 nation of 90, the concavity disappears altogether, under circumstances analogous 

 to those already described. 



If, in place of using an inverting eye-piece, we invert the concavity m A n, by 



* The inversion of an object never makes the nearer part of an object more remote, nor the re- 

 mote part nearer. 



