in connexion with Vision. 249 



situated in the axis of vision, which is more acutely sensible 

 to the rays of light than any other part; and the eyeball is 

 admirably provided to turn and present its transparent surface 

 so that it may catch the impressions of objects upon this spot. 

 It is only when images fall upon this part of the retina that 

 the mind is satisfied, or that a perfect sensation is obtained ; 

 and the power of guiding the eye with this intention, whether 

 it be through the motions of the body or of the head, or is 

 produced directly by the action of the muscles of the eye, is 

 as necessary to the perception of the direction of the things 

 around us, as the groping about with the hands is to a blind 

 person, to enable him to. find his way through the confused 

 furniture of a room. We are sensible of this searching motion 

 of the eye, made in preparation for distinct vision ; and it is 

 by calculating the extent of this motion that we estimate 

 * direction.' 



In common language we are accustomed to speak of ' direct* 

 ing' the eye : it is allowed also, that it is by a voluntary act 

 that we accomplish this ; but it is not so generally conceded 

 that the exercise of the muscles, by which the eye is moved, 

 communicates a distinct sensation to the mind ; and yet it 

 forms an important part in the process of vision. When an 

 astronomer directs his telescope to the heavens, he is aware 

 that his instrument combines in it two separate means of ac- 

 quiring the knowledge which he seeks. By the apparatus of 

 lenses or mirrors inclosed within the tube, he ascertains the 

 magnitude, brilliancy, or colour of the star ; but it is by a 

 perfectly distinct apparatus that he determines its place in the 

 heavens. He consults, for this purpose, the external parts of 

 his instrument, such as the various levers, screws, and joints 

 by which the tube is revolved ; and it is by looking to the scale 

 attached to his telescope, that he is informed of the exact 

 bearing or of the direction of the particular star. Now the 

 outward apparatus of muscles by which the eyeball is directed 

 in vision, is surely, in an inquiry of this kind, deserving of at- 

 tention. There is a sensibility resident in the muscles which 

 gives token of the degree of their contractions ; and it is by 

 attending to these impressions, like looking to the scale of the 

 telescope, that we become conscious of the direction of objects. 



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