SENSATION IS GENERAL. 39 



move matter from place to place, and change its 

 appearances, that are the original sources of all our 

 knowledge of the mechanical properties of matter. 

 In common language, indeed, we are accustomed 

 to say, that we measure visible distance by the 

 eye, and the distance of sound by the ear ; but it 

 is exceedingly probable, nay almost certain, that 

 the origin of our knowledge in those cases is in 

 our muscles, our organs of motion ; and that, even 

 in the case of the eye itself, which is the organ 

 that we can best understand, and most nearly imi- 

 tated by artificial contrivances, it is the muscular 

 action by which it is adapted to different distances, 

 and not the degree of light, or the magnitude, or 

 intensity of the picture formed on the retina of 

 the eye, that gives us notions of distance and also 

 of magnitude. The formation of that picture, 

 though, as there is little doubt it does, it takes 

 place in the living eye, is, after all, merely a me- 

 chanical matter; and any one can produce it at 

 pleasure, by closing the shutters, boring a hole in 

 them, placing a glass which is convex or thick in 

 the middle, in the opening, and holding a sheet of 

 paper at the proper distance behind. Not only 

 that, but, by means of mirrors properly placed, or 

 prisms of glass, which reflect from their hinder 

 surfaces, we can convey those images of visible 

 things whithersoever we will. That beautiful con- 

 trivance of Ramsden's, which, from being in itself 

 invisible, is called " Ramsden's ghost," is a re- 

 markable instance of that. In a fine astronomical 

 instrument for taking the elevations of the celestial 

 bodies, it is necessary that the plummet should, by 

 means of the spider's thread, or whatever other 

 delicate substance is used for marking it, pass in a 

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