SENSE OF VISION. 419 



vented from perceiving that it is but a picture. In this case, however, a slight 

 movement of the head is sufficient to undeceive us ; since by this movement 

 a great change would be occasioned in the perspective view of the object, sup- 

 posing it to possess an uneven surface ; whilst it scarcely affects the image 

 formed by a picture. In the same manner, a person who only possesses one eye, 

 obtains, by a slight motion of his head, the same idea of the form of a body, 

 which another would acquire by the simultaneous use of his two eyes. 



548. The appreciation of the distance of objects, may be easily shown to 

 be principally derived from the association, in the mind, of visual and tactual 

 sensations ; assisted, in regard to near objects, by the muscular sensations de- 

 rived from the convergence of the eyes. Thus, an infant, or a person who 

 has but recently acquired sight, evidently forms very imperfect ideas regarding 

 the distance of objects ; and it is only after long experience that a correct no- 

 tion is formed. The assistance which is given by the joint use of both eyes, 

 is evident from the fact, that, if we close one eye, we are unable to execute 

 with certainty many actions, which require a precise appreciation of the dis- 

 tance of near objects, such as threading a needle, or snuffing a candle. In 

 regard to distant objects, our judgment is chiefly founded upon their apparent 

 size, if their actual size be known to us ; but, if this is not the case, and if we 

 are so situated that we cannot judge of the intervening space, we principally 

 form our estimate from the greater or less distinctness of their colour and out- 

 line. Hence this estimate is liable to be greatly affected by varying states of 

 the atmosphere ; as is well known to every one who has visited warmer lati- 

 tudes. The extreme clearness of the air sometimes brings, into an apparently 

 near proximity, a hill that rises beyond some neighbouring ridge (the inter- 

 vening space being hidden, so as not to afford any datum for the estimate of 

 the distance of the remote hill) ; and which, by a slight haziness, is carried to 

 three or four times the degree of apparent remoteness. It is probable that, in 

 the lower Animals, the perception of distance is much more intuitive than it is 

 in ourselves. 



549. Our estimate of the real size of an object is manifestly connected with 

 that of its distance. The apparent size is dependent upon the angle at which 

 its rays diverge, to impinge upon the cornea ; this angle increases with the 

 proximity, and diminishes with the remoteness, of the object. Our estimate 

 of the comparative size of near objects, of whose distances we can become 

 aware by the inclination of the optic axes, is much more correct than that 

 which we form, when one or both are far removed; since, when we are un- 

 certain as to its distance, we cannot form a judgment of the real size of a body, 

 from the angle at which its rays diverge. Hence our estimate of the size of 

 objects even moderately distant, is much influenced by states of the atmosphere. 

 Thus, if we walk across a common in a fog, a child approaching us appears 

 to have the size of a man, and a man seems like a giant; since the indistinct- 

 ness of the outline excites in the mind the idea of distance; and an object 

 seen under a given visual angle at a distance, must of necessity be much larger 

 than one, of which the apparent size is the same, but which is much nearer. 

 The want of innate power in Man to form a true conception of either size or 

 distance, is well shown by the effect produced on the mind unprepared for 

 such delusions, by a skilfully-painted picture; the view of which is so con- 

 trived, that its distance from the eye cannot be estimated in the ordinary man- 

 ner; the objects it represents are invested by the mind with their real sizes 

 and respective distances, as if their real image was formed upon the retina.* 



> This delusion has been extremely complete, in some of those who have seen the pano- 

 ramic view of London in the Coliseum. A lively and interesting account of it is given in 

 the Journal of the Parsee Shipbuilders, who visited England some time ago. 



