506 



SCIENTIFIC THOUGHT. 



a kind of deception, gave to perfectly flat surfaces the 

 vivid appearance of depth and distance. And here we 

 may note, in passing, how it was almost entirely left 

 to foreign thinkers to utilise this remarkable invention 

 for the benefit of the theory of vision and the science 

 of psycho -physics ; l "VVhewell having characteristically 

 omitted this epoch-making fact, as in his well-known 

 history he omitted to notice many other contemporary 

 British contributions to science. 



Philosophers, who are accustomed to find hidden 

 problems where ordinary persons only see common-sense, 

 had already approached the question of the genesis of our 

 space perception from two definite points of view, which 

 we may, for the sake of convenience, identify with the 

 names of Kant and Herbart. The genetic view associ- 

 ated by the physiologists with the name of Kant, and 

 supposed to have been prepared by Locke, Berkeley, and 

 Hume, was this, that what we know of external things 

 depends upon the peculiarities of our own perceiving 



1 Sir Charles Wheatstone (1802- 

 1875), to whom several inventions 

 of equal scientific and practical 

 interest are due, invented the 

 mirror - stereoscope in 1833. A 

 notice of it was given in Mayo's 

 ' Outlines of Human Physiology,' 

 but neither its theoretical nor its 

 practical importance was recognised 

 till Wheatstone published his paper 

 in the 'Phil. Trans.' in 1838. He 

 there refers to Leonardo da Vinci as 

 having been the only one before him 

 to notice the difference of binocular 

 and monocular vision. Since Wheat- 

 stone's invention became known and 

 was perfected by Brewster, Moser, 

 and others, and especially since 

 Helmholtz entered the field with his 

 extensive and original researches in 



optics, it has been found that 

 ancient as well as more recent 

 philosophers had approached the 

 subject very closely ; and many 

 references are given in the new 

 edition of the ' Physiologische 

 Optik ' (1896), p. 840. The inven- 

 tion of photography about the same 

 time (1835, by Daguerre, after ex- 

 tensive and prolonged experiments 

 by himself and Niepce, published in 

 1839 by Arago), which was of great 

 importance to optical theory, was 

 also for some time singularly little 

 appreciated by theorists. See 

 Rosenberger, ' Gesch. d. Physik,' 

 vol. iii. p. 316. See also Helmholtz's 

 lecture "Ueber das Sehen des 

 Menschen" (1855). 



