252 OF SENSATION, AND THE ORGANS OF THE SENSES. 



the two eyes is so slight, that it cannot aid in the determination ; and hence 

 it is, that,\vhilst we have no difficulty in distinguishing a picture, however 

 well painted, from a solid object, when placed near our eyes, (since the idea 

 which might be suggested by the image formed on one eye, will then be 

 corrected by the other,) we are very liable to be misled by a delineation, in 

 which the perspective, light and shade, &c., are faithfully depicted, if we are 

 placed at a distance from it, and are prevented 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, supposing it to possess an uneven sur- 

 face ; 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. 



341. 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 

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

 has but recently acquired sight, evidently forms very imperfect ideas regard- 

 ing the distance of objects ; and it is only after long experience that a correct 

 notion 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 exe- 

 cute with certainty many actions which require a precise appreciation of the 

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

 In regard to distant objects, our judgment is chiefly founded upon their appa- 

 rent 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 prin- 

 cipally form our estimate from the greater or less distinctness of their colour 

 and outline. 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 latitudes. The extreme clearness of the air sometimes brings into an 

 apparently near proximity a hill that rises beyond some neighbouring ridge 

 (the intervening space being hidden, so as not to afford any datum for the esti- 

 mate 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 pro- 

 bable that, in the lower animals, the perception of distance is much more 

 intuitive than it is in ourselves. 



342. Our estimate of the real size of an object is manifestly connected Avith 

 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 uncer- 

 tain 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 atmo- 

 sphere. 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 

 indistinctness 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 unpre- 

 pared for such delusions, by a skilfully-painted picture ; the view of which is 



