SENSE OF VISION. 247 



power of distinguishing minute parts of that surface ; since, as Weber justly 

 remarks, two impressions falling upon one of these points, can scarcely affect 

 the sensorium otherwise than with one sensation. 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 individuals 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 other- 

 wise distinguish them ; and the same is the case with persons of acuter per- 

 ception, with respect to objects at distances greater than those at which they 

 can see most clearly. " 1 myself," says Ehrenberg, " cannot see ^yL-^th 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 bal- 

 loon, 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 

 ( 313). 



334. 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, dis- 

 tances, &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 w r orld, 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 every thing^fotf, as in a picture ; simply receiving the con- 

 sciousness of the impressions made upon his retina : and it was some time 

 before 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 

 remember 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 

 associate the two sets of ideas ; and then, setting her down, said, " So, puss, I 

 shall know you another time." A similar instance has come under the 

 Author's own knowledge ; but the subject of it was scarcely old enough to 

 present phenomena so striking. One curious circumstance was remarked of 

 him, which fully confirms (if confirmation were wanting) the view here given. 



