MIND AS A MEASURE OF NATURE. 689 



It is well known that but a portion of the phenomena of nature 

 are cognizable by us how small a portion it is not known. A person 

 born blind has no conception of color ; one born deaf, none of sound. 

 The disease of color-blindness affords a good illustration of the limita- 

 tion of knowledge in consequence of the limitation of sensibility in 

 certain directions. One affected with this disease, while able to see 

 outlines with perfect distinctness, as well as some of the colors, is 

 insensible to others ; and this insensibility does not affect alone the 

 ability to discriminate between colors nearly alike, but also those that 

 are apparently widely divergent. It is related of Dalton, from whom 

 color-blindness takes its name of daltonism, that some wag exchanged 

 the robe of sober drab which this demure Quaker was in the habit 

 of wearing, for one of scarlet, and that, unconscious of the differ- 

 ence, he went forth, much to the amusement of the bystanders. Un- 

 doubtedly many accidents are due to the inability to discriminate 

 between different colored signals. This subject is just beginning to 

 receive the attention its importance demands, and it is found much 

 more prevalent than was even suspected. Like short-sightedness and, 

 probably, all maladies due to the absence or the imperfection of any 

 of the senses, the subject of it is unconscious of its existence until 

 something particular calls it to his attention. To a greater or less ex- 

 tent we may all be said to be color-blind. Only a part of the rays 

 that are known to proceed from the sun are visible to any of us. 

 Vision is produced only by waves of a certain definite length. The 

 rays of longer wave-length than the red are known to us by their heat- 

 producing effects only ; those of shorter wave-length than the violet, 

 by their chemical effects. It is not improbable that, at some future 

 time, a race may exist capable of seeing rays invisible to us just as 

 we perceive many invisible to some of the lower animals. In the de- 

 velopment of sight there must, at first, have been simply a power to 

 distinguish light from darkness ; then, the ability to distinguish out- 

 lines ; and, finally, the sensibility to color. Until this sensibility was 

 developed, objects would present merely an alternation of light and 

 shade, similar to the view we get in a stereoscopic picture. 



Sounds inaudible to us are heard by insects, and odors that we 

 can not distinguish are smelled by animals. So we might go through 

 the whole category of sensations and resulting ideas, and show their 

 variation imder different conditions ; and yet, whatever may be the 

 theory of knowledge we espouse, it is from these sensations and ideas 

 that we form our conception of nature. How different our mental 

 conceptions would be if our senses were indefinitely magnified, we 

 can but guess. A drop of water, it has been estimated by Sir William 

 Thomson, if magnified to the size of the earth, would be seen to con- 

 sist of molecules of the size of cricket-balls ; and these molecules are 

 themselves of great complexity. The internal motions of a drop of 

 blood are more complex than the motions of the solar system. Place 



YOL. XTIII. 44 



