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hand, a pair of pictures slightly dissimilar is seen when the optic 

 axes converge very much, the appearance is that of a bas-relief. 

 As no disagreeable or obviously incongruous effect is produced when 

 two pictures, intended for a nearer convergence of the optic axes, 

 are seen when the eyes are parallel or nearly so, we are able to avail 

 ourselves of the means of augmenting the perceived magnitude of 

 the binocular image mentioned at the commencement of this abstract. 

 For this purpose the pictures, placed near the eyes, are caused to 

 coincide when the optic axes are nearly parallel ; and the diverging 

 rays proceeding from the near pictures are rendered parallel by 

 lenses of short focal distance placed before the mirrors or prisms of 

 the stereoscope. 



Some additional observations are next brought forward respect- 

 ing those stereoscopic phenomena which the author, in his first 

 memoir, called " conversions of relief." They may be produced in 

 three different ways -1st, by transposing the pictures from one eye 

 to the other ; 2ndly, by reflecting each picture separately, without 

 transposition ; and 3rdly, by inverting the pictures to each eye se- 

 parately. The converse figure differs from the normal figure in this 

 circumstance, that those points which appear most distant in the 

 latter, are the nearest in the former, and vice versd. 



An account is then given of the construction and effects of an 

 instrument for producing the conversion of the relief of any solid 

 object to which it is directed. As this instrument conveys to the 

 mind false perceptions of all external objects, the author calls it a 

 Pseudoscope. It consists of two reflecting prisms, placed in a frame, 

 with adjustments, so that, when applied to the eyes, each eye may 

 separately see the reflected image of the projection which usually 

 falls on that eye. This is not the case when the reflexion of an 

 object is seen in a mirror; for then, not only are the projections 

 separately reflected, but they are also transposed from one eye to 

 the other, and therefore the conversion of relief does not take place. 

 The pseudoscope being directed to an object, and adjusted so that 

 the object shall appear of its proper size and at its usual distance, 

 the distances of all other objects are inverted ; all nearer objects ap- 

 pear more distant, and all more distant objects nearer. The con- 

 version of relief of an object consists in the transposition of the 

 distances of the points which compose it. 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 relations with each other. Among the remarkable illusions 

 it occasions, the following are mentioned. The inside of a tea- 

 cup appears 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, appears to be a vertical section of the in- 

 terior of the vase, with painted hollow impressions of the flowers. 

 A small terrestrial globe appears a concave hemisphere ; when the 

 globe is turned on its axis, the appearance and disappearance of 

 different portions of the maj) on its concave surface has a very sin- 

 gular effect. A bust regarded in front becomes a deep hollow 



