58 INNEBVATION. [cHAP.XVii. 



as regards the body our several organs are conceived as exist- 

 ing in a particular relative position, altogether independently of 

 vision and vision accords entirely, and at once, with these senses 

 in the determination of locality, without the necessity of an educa- 

 tion of the sense, such as a reversed impression on the mind through 

 the eye would require. 



Were the eye and the whole body fixed, we should still have 

 a knowledge of the relative position of visible objects, and of course 

 of the direction in which each point of their surface was placed, as 

 regards the organ of sense; and as rays coming from objects in the 

 same direction as regards the body, would then always fall on the 

 same part of the retina, we might conclude that each part of that 

 membrane had the power of conveying the notion of position in 

 one direction only as regards the body. But the eye being a very 

 moveable organ, we are enabled to make the image of a stationary 

 object travel successively over a large tract of the retina without the 

 object appearing to move; since we are conscious, through the mus- 

 cular sense, of the motion in our own eye. The visual idea of di- 

 rection in regard to the body, therefore, does not depend on the 

 image falling on a particular point of the retina, but in a great 

 measure on the muscular sense, in conjunction with that quality of 

 the excitability of the retina already spoken of. 



It is proper also to mention that the limits of the field of vision, 

 formed by certain parts of the face, are a standard to which the 

 mind refers in estimating the position of visible objects. The out- 

 line of the field remains the same, through all movements of the 

 eye. The motions of the head or body can alone bring new objects 

 into the field ; and the muscular sense thus still further contributes 

 to enhance the usefulness of the sense of vision. 



Visual Perception of Shape and Size. If an object form a 

 large image on the retina, and of a square figure, we conceive it at 

 once to be large and square ; and of this no other explanation can 

 be given than that the visual points making up the surface of the 

 retina have, as regards space, a relation to one another, of which the 

 mind is intuitively cognizant in framing its ideas from visual im- 

 pressions. But the size of an image, relatively to the whole retina, 

 will vary with the distance of the object; and the conception of its 

 real dimensions would be erroneous, were it not that the impression 

 were corrected by the muscular sense engaged in the adjustment of 

 the eye to distance, and by the lessons of experience. When a 

 person, blind by cataract from infancy, is couched, he concludes 

 that the diversified details of the scene presented to him are at an 



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