518 Prof. Wheatstone on the Physiology of Vision. 



which result from combining these elements inversely ; so that 

 as an object becomes nearer, its larger picture on the retina is 

 accompanied by a less convergence of the optic axes. With the 

 pseudoscope we have a glance, as it were, into another visible 

 world, in which external objects and our internal perceptions 

 have no longer their habitual relation with each other. 



I wfll now proceed to describe some of the illusions produced 

 by the aid of this instrument. Those which may be strictly 

 designated conversions of relief, in which the illusive appearance 

 has the same relation to that of the real object as a cast to a 

 mould, or a mould to a cast, are very readily perceived. I must 

 however remark, that it is necessary to illuminate the object 

 equally, so as to allow no lights or shades to appear upon them, 

 for their presence has a considerable influence on the judgement, 

 and is one of the principal causes of the perception of the proper 

 relief when a single eye is employed. 



The inside of a tea-cup appears as a solid convex body; the 

 effect is more striking if there are painted figures within the cup. 



A china vase, ornamented with coloured flowers in relief, pre- 

 sents a very remarkable appearance ; we apparently see a vertical 

 section of the interior of the vase, with painted hollow impres- 

 sions of the flowers. 



A small terrestrial globe appears as a concave hemisphere ; 

 on turning it round on its axis, it was curious to see different 

 portions of the spherical map appear and disappear in a manner 

 that nothing in external nature can imitate. 



A bust regarded in front becomes a deep hollow mask ; the 

 appearance when regarded in profile is equally striking. 



A framed picture hanging against a wall, appears as if im- 

 bedded in a cavity made in the wall. 



A medal, or the impression of a seal, is perfectly converted 

 into a representation of the die from which it has been struck ; 

 and, on the other hand, the mould or die of a medal, or an en- 

 graved seal, becomes a facsimile of the medal or raised impres- 

 sion. It will also be observed, that if the medal be placed on a 

 flat surface, as a sheet of paper, it will appear sunk beneath the 

 surface ; and if it be placed in a hollow of the same size, it will 

 appear to stand above the surface as much as it actually is 

 below it. 



These appearances are not always immediately perceived ; and 

 some much more readily present themselves than others. Those 

 converse forms which have a meaning, and resemble real forms 

 we have been accustomed to see, are those which are the most 

 easily apprehended. Viewed with the pseudoscope, notwithstand- 

 ing the inversion of the pictures on the retina, the natural ap- 

 pearance of the object continues to intrude itself, when some- 



