PERCEPTION OF BULK AND FOltM. 



92. Sight does not afford any immediate perception either of the 

 volume or shape of an object. The information which we derive 

 from the sense, of the bulk or figure of distant objects, is obtained 

 by the comparison of different impressions made upon the sense of 

 sight by the same object at different times and in different posi- 

 tions. A body of the spherical form seen at a distance appears to 

 the eye as a flat circular disk, and would never be known to have 

 any other form, unless the impression made upon the eye were 

 combined with other knowledge, derived from other impressions, 

 through sight or touch, or both these senses, and thus supplied 

 the understanding with data from which the real figure of the 

 object could be inferred. The sun appears to the eye as a flat, 

 circular disk ; but, by comparing observations made upon it at 

 different times, it is ascertained that it revolves round one of its 

 diameters in a certain time, presenting itself under aspects in- 

 finitely varying to the observer ; and this fact, combined with its 

 invariable appearance as a circular disk, proves it to be a sphere ; 

 for no body except a sphere, viewed in every direction, would 

 appear circular. 



Although we do not obtain from the sense of sight a perception 

 of the shape of a body, we may obtain a perception of the shape 

 of one of its sections. Thus, if a section of the body be made by 

 a plane passing through it at right angles to the line of vision, 

 the sight supplies a distinct perception of the shape of such section. 

 Thus, if an egg were presented to the eye with its length in the 

 direction of the line of vision, it would appear circular, because a 

 section of it made by a plane at right angles to its length is a 

 circle ; but if it were presented to the eye with its length at right 

 angles to the line of vision, it would appear oval, that being the 

 shape of a section made by a plane passing through its length. 



If a body, therefore, presents itself successively to the eye in 

 several different positions, we obtain a knowledge by the sense of 

 sight of so many different sections of it, and the combination of 

 these sections may in many cases supply the reason with data by 

 which the exact figure of the body may be known. 



93. As the term " apparent magnitude " is used to express the 

 visual angle under which an object is seen, we shall adopt the 

 term visible area to express the apparent magnitude of the section 

 of a visible object made by a plane at right angles fo the line of 

 vision, that is to say, to the line drawn from the eye to the centre 

 of the object. 



94. Besides receiving through the sight a perception of the 

 figure of the section of the object which forms its visible area, we 

 also obtain a perception of the lights and shades and the various 

 tints of colour which mark and characterise such area. By 



93 



