414 ON SENSATION, AND THE ORGANS OF THE SENSES. 



least of the foregoing ; thus, gold dust* of the fineness of 1-1 125th of an inch, 

 may be discerned with the naked eye in common daylight. The delicacy of 

 vision is far greater for lines than for single articles ; opaque threads of l-4900th 

 of an inch in diameter may be discerned with the naked eye, when held to- 

 wards the light. Such threads are about half the diameter of the Silk-worm's 

 fibre. The degree in which the attention is directed to them, has a great 

 influence on the readiness with which very minute objects can be perceived; 

 and Ehrenberg remarks that there is a much greater difference amongst indi- 

 viduals in this respect, than there is in regard to the absolute limits of vision. 

 Many persons can distinctly see such objects, when their situation is exactly 

 pointed out to them, who cannot otherwise distinguish them; and the same 

 is the case with persons of acuter perception, with respect to objects at dis- 

 tances greater than those, at which they can see most clearly. " I myself," 

 says Ehrenberg, " cannot see l-2700th of an inch, black on white, at twelve 

 inches distance; but having found it at from four to five inches distance, I 

 can remove it to twelve inches, and still see the object plainly." Similar 

 phenomena are well known in regard to a balloon, or a faint star, in a clear 

 sky; or a ship in the horizon: we easily see them after they have been 

 pointed out to us; but the faculty of rapidly descrying depends on the habit 

 of using the eyes in search of such objects ( 519). 



541. The sense of Vision depends, in the first place, on the transference 

 to our minds of the picture which is formed upon the retina ; this picture puts 

 us in possession of the outlines, lights and shades, colours, and relative posi- 

 tions of the objects before us ; and all the ideas respecting the real forms, 

 distances, &c., of bodies, which we found upon these data, must be considered 

 in the light of perceptions, either instinctive or acquired. Many of these are 

 derived through the combination, in our minds, of the Visual sensations, with 

 those derived from the sense of Touch. Thus, to take a most simple illustra- 

 tion, the idea of smoothness is one essentially tactile ; and yet it constantly 

 occurs to us, on looking at a surface which reflects light in a particular man- 

 ner. But, if it were not for the association, which experience leads us to 

 form, of the connection between polish as seen by the eye, and smoothness 

 as felt by the touch, we should not be able to determine, as we now can do, 

 the existence of both these qualities from an impression communicated to us 

 through either sense singly. The general fact that, in Man, the greater part 

 of those notions of the external world by which his actions in the adult state 

 are guided, are acquired by the gradual association of the sensations commu- 

 nicated by the sight and by touch, is substantiated by amply-sufficient evidence. 

 This evidence is chiefly derived from observations made upon persons born 

 blind, to whom sight has been communicated by an operation, at a period of life 

 which enabled them to give an accurate description of their sensations. The 

 case recorded by Cheselden is one of the most interesting of these. The 

 youth (about 12 years of age), for some time after tolerably distinct vision had 

 been obtained, saw everything Jlat, as in a picture ; simply receiving the con- 

 sciousness of the impression made upon his retina ; and it was some time be- 

 fore he acquired the power of judging, by his sight, of the real forms and 

 distances of the objects around him. An amusing anecdote recorded of him, 

 shows the complete want of natural or intuitive connection which there is in 

 Man, between the ideas formed through visual and through tactile sensations. 

 He was well acquainted with a Dog and a Cat by feeling ; but could not re- 

 member their respective characters when he saw them. One day, when thus 

 puzzled, he took up the Cat in his arms, and felt her attentively, so as to as- 



* Ehrenb?rg mentions thnt he obtained the finest particles of gold, by scraping gilt brass ; 

 by filing pure gold, he always obtained much coarser particles. 



