XIX 



so placed that they might be seen each by one eye, but visually super- 

 posed, either by reflection or by refraction \ this he effected by a combi- 

 nation of lenses either with plane reflectors or with prisms. All that is 

 really due to Brewster in relation to the stereoscope is the use of wedge- 

 shaped segments of larger lenses, in which the lens and prism of Wheat- 

 stone are combined ; the construction now universally adopted, and popu- 

 larized by the application of photography. 



In 1852 Wheatstone presented a second memoir on Binocular Vision 

 to the Eoyal Society, in which he produced a striking confirmation of his 

 former views by the invention of the Pseudoscope, an instrument in which 

 the perspective pictures of a given object as seen by the two eyes respec- 

 tively are reversed by internal reflection from the bases of two right- 

 angled prisms, and convex objects appear to be concave, or vice versa ; 

 thus a globe on which figures are traced will appear to be concave, and 

 the inside of a painted basin or saucer to be convex, or the object as it 

 were turned inside out. 



The use of this instrument has produced some curious illustrations of 

 mental phenomena : with most persons an object, of which the convexity 

 or concavity is unknown, will at once appear inverted ; but when the real 

 form is known, with some persons experience appears to overrule visual 

 perception, and they cannot succeed in seeing the inversion of the object ; 

 with others the appearance derived from previous knowledge persists for 

 a shorter or longer period, and then suddenly gives way to the visual 

 impression, and the object appears to be suddenly turned inside out; 

 while with others, again, the change is more or less gradual, and during 

 the period of transition the visual impression is uncertain and indistinct. 

 In this memoir the idea of the mental character of binocular vision 

 was still further supported by experimental evidence that the apparent 

 magnitude of objects is governed by the relative position as well as the 

 size of the visual pictures. In fact the whole of these investigations may 

 be viewed in their relation to mental conditions ; and when thus regarded, 

 they possessed a higher interest than that which arose from their great 

 value as contributions to physiological optics. 



At one period, indeed, Wheatstone's attention was for a time directed 

 to problems of mental philosophy, and especially to the gi^m-mechanical 

 solution of some of them, which was hoped for by the followers of Gall 

 and Spurzheirn ; he was an active member of the London Phrenological 

 Society, then presided over by Dr. Elliotson ; and in January 1832 he 

 read at one of the meetings a paper on Dreaming and Somnambulism, 

 which was published in ecotenso in the ' Lancet ' of that date. This paper 

 is remarkable, like all his writings, for the extreme clearness with which 

 known facts are stated, and the deductions based upon them. 

 - Amongst "Wheatstone's many ingenious inventions may be mentioned 

 his automatic apparatus for recording periodically the height of the baro- 

 meter, and the temperatures of the dry- and wet-bulb thermometers, by 



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