694 THE POPULAR SCIENCE MONTHLY. 



feel " hot " and those that feel " cold ; " and in this manner we arrive 

 at the notion of differences of temperature. And it is through the me- 

 dium of our tactile sense, without any aid from vision, that we first 

 gain the idea of solid form, or the three dimensions of space. 



Again, hy the extension of our tactile exjjeriences, we acquire the 

 notion of liquids, as forms of matter yielding readily to pressure, but 

 possessing a sensible weight which may equal that of solids ; and of 

 air, whose resisting power is much slighter, and whose weight is so 

 small that it can only be made sensible by artificial means. Thus, 

 then, we arrive at the notions of resistance and of weight as properties 

 common to all forms of matter; and, now that we have got rid of that 

 idea of light and heat, electricity and magnetism, as " imponderable 

 fluids," which used to vex our souls in our scientific childhood, and of 

 which the popular term "electric fluid" is a " survival," we accept 

 these properties as affording the practical distinction between the 

 " material " and the " immaterial." 



Turning, now, to that other great portal of sensation, the sight, 

 through which we receive most of the messages sent to us from the 

 universe around, we recognize the same truth. Thus it is agreed, alike 

 by physicists and physiologists, that color does not exist as such in the 

 object itself; which has merely the power of reflecting or transmitting 

 a certain number of millions of undulations in a second ; and these 

 only produce that affection of our consciousness, which we call color, 

 when they fall upon the retina of the living percipient. And if there 

 be that defect, either in the retina or in the apparatus behind it, which 

 we call " color-blindness " or Daltonism, some particular hues cannot 

 be distinguished, or there may even be no power of distinguishing any 

 color whatever. If we were all like Dalton, we should see no differ- 

 ence, except in form, between ripe cherries hanging on a tree and the 

 green leaves around them ; if we were all affected with the severest 

 form of color-blindness, the fair face of Nature would be seen by us as 

 in the chiaroscuro of an engraving of one of Turner's landscapes, not 

 as in the glowing hues of the wondrous picture itself. And, in 

 regard to our visual conceptions, it may be stated with perfect cer- 

 tainty, as the result of very numerous observations made upon persons 

 who have acquired sight for the first time, that these do not serve for 

 the recognition even pf those objects with which the individual had 

 become most familiar through the touch until the two sets of sense- 

 perceptions have been coordinated by experience. 1 When once this 

 coordination has been effected, however, the composite perception of 



1 Thus, in a recently-recorded case in which sight was imparted by operation to a 

 young woman who had been blind from birth, but who had, nevertheless, learned to work 

 well with her needle, when the pair of scissors she had been accustomed to use was 

 placed before her, though she described their shape, color, and glistening metallic charac- 

 ter, she was utterly unable to recognize them as scissors until she put her finger on them ; 

 when she at once named them, laughing at her own stupidity (as she called it) in not 

 having made them out before. 



