♦34 



Sir David Brewster oji the 



the field of view that strikes the table, and rests upon it. With 

 these convictions, he sees what is represented in fig. 4'. The 

 concavity m A v, 

 fig. 3, appears in- 

 verted ; and as the 

 visible part of the 

 concavity Ain, fig. 

 3, is nearesttheeye 

 infig.-t, and the in- 

 visible part Ay/, fig. 



3, furthest from the eye in fig. 4, 7n A n must appear 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 



Fig. 5. 



All these observations are 



which springs from the other 

 The^rsl illusion is the belief 

 that the surface M N is look- 

 ing upwards, whereas it is 

 really inverted, as shown in 

 fig. 5 ; and the second illusion, 

 which arises from the first, is, 

 that the pomtjiappearsjiut/iest 

 from the eye, whereas it is 

 ncm-cst to it, as shown in fig, 5. 

 equally applicable miUalis 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 produced 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 N O of the stripe of 

 wood distinctly seen, the inversion of the surface M N, which 

 now looks downward, will l)e at once recognised. The edge 

 71 of the concavity will appear nearest the eye*, as it really is, 

 and t]ie concavity, though inverted, "will still appear a concavity. 

 The very same reasoning is applicable to a convexity on a 

 narrow stripe of wood. 



* The inversion of an object never makes the nearer part of an object 

 viore remote, nor the remote part nearer. 



