296 ADDITIONAL SENSES. 



such as to cause the figures, drawn on both sides of the card, to be seen 

 at nearly the same time. The phantasmascope, phenakistiscope and 

 anorthoscope, act upon similar principles. 1 



It is by accurate attention to various optical illusions, and to the 

 laws of the animal economy on which they are founded, that many of 

 them can be produced in the arts at pleasure. Painting is, in truth, 

 little more than depicting on canvass the various optical errors, which 

 we are habitually incurring. 



To conclude : the sense of sight differs materially in the scale of 

 animals : in few is the organization more perfect or the function better 

 executed than in man. Situate at the upper and anterior part of the 

 body, the organ of vision is capable of directing its regards over a large 

 extent of surface: the axes of the two organs can be converged upon 

 objects in various situations, which cannot be done by many animals; 

 and they are very movable under the domination of a muscular appa- 

 ratus of admirable arrangement. Still, the eye is not as delicately 

 organized as in some animals, which are capable of seeing objects at a 

 distance that would be totally beyond the reach of the visual powers of 

 man. 



Like the- other senses, sight can be exerted actively and passively ; 

 hence the difference between simply seeing, and looking. In the latter, 

 the eye is directed to the object by the proper muscles; and it is not 

 improbable, that the nerve may be aroused to a more accurate and deli- 

 cate reception of impressions, as we have reason for believing is the 

 case in the other senses. Like them, it admits of great improvement 

 by education. The painter, and the worker in colours are capable of 

 nice discrimination, and detect ,the minutest shades of difference with 

 great facility. In savage life, where the tracks or marks through the 

 almost interminable forests, or over the pathless wilds, are the only 

 guides, the greatest acuteness of vision is necessary ; and, accordingly, 

 we find the North American Indian, in this respect, eminently distin- 

 guished. The mariner, too, accustomed to look out for land, or for a 

 hostile sail, detects it in the distant horizon long before it can be per- 

 ceived by the landsman, and appreciates its distance and course with 

 signal accuracy, education, in this case, not only communicating to 

 his eye facility in being impressed, but improving the intellectual pro- 

 cess, by which the estimation of distances is arrived at. 



ADDITIONAL SENSES. 



The five senses constitute so many special nervous systems, each 

 concerned in its appropriate function; and, although conveying ideas of 

 the external world to the brain, and connected with that organ, they are 

 to a certain extent independent of it. The generality of physiologists 

 admit these five only; but some have suggested others, differing, in 

 general, from the five, in having no orgJn at the surface of the body 

 exclusively concerned in the function. Buffon regarded as a sixth sense 

 the intense sensation experienced during the venereal act; but this can 



1 Muller, Principles of Physics and Meteorology, p. 310, Philad., 1848. 



