888 OF SENSATION, AND THE ORGANS OP THE SENSES. 



of both eyes, is made evident by the fact that, if we close one eye, we find our- 

 selves unable to execute with certainty many actions (such as threading a needle 

 or snuffing a candle) which require its guidance; and we can scarcely conceive 

 of any other basis for this appreciation than that which is afforded by the mus- 

 cular sensations produced by the different degrees in which the optic axes are 

 made to converge, according to the distances of the objects to which we direct 

 our eyes. For, in proportion as they are removed further and further, do the 

 optic axes approach parallelism, and the power of appreciating differences of 

 distance is lost; whilst, on the other hand, in proportion as the object is ap- 

 proximated to the eyes, slight differences of distance produce marked differences 

 in the degree of convergence; and these are readily appreciated, so as to afford 

 the means of very nice discrimination. The large extent to which our notion 

 of the relative distances of near objects is due to variations in the angle of con- 

 vergence of the optic axes, is further shown by the following experiment devised 

 by Prof. Wheatstone. If two similar pictures be placed in his mirror-stereoscope, 

 and be made to move to and from the mirrors, so as to vary their distances from 

 these, and therefore from the eyes, without altering the angle of convergence, 

 their apparent sizes are seen to change (in consequence of the alteration of the 

 visual angle), but no positive change is seen in their apparent distances ; the 

 effect produced being very much like that of the enlargement or diminution of 

 the images on the screen in the exhibition of the Phantasmagoria, suggesting the 

 idea of approach or recession, although we perceive that the distance of the 

 screen from our eyes has undergone no alteration. A converse effect, as we 

 shall presently see ( 890), is produced by alterations in the angle of converg- 

 ence, without any real change in the distance of the pictures. This power of 

 estimating distance, however is obviously, in Man, not an intuitive but an ac- 

 quired endowment ; for it is evident to any observer that infants, or older per- 

 sons who have but recently acquired sight, form very imperfect ideas respecting 

 the distance of objects, their attempts to grasp bodies which attract their atten- 

 tion being for a long time unsuccessful ; and that they only gradually learn to 

 measure distances by the sight through the medium of the touch. It is pro- 

 bable, however, that, in the lower Animals, especially such as have early to rely 

 upon their own exertions for the supply of their natural wants, the perception 

 of distance is more intuitive than it is in ourselves ; since we may observe them 

 very early performing actions which require an exact appreciation of it. 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 be not the case, and if we are so 

 situated that we cannot judge of the intervening space, we principally form our 

 estimate from that effect of different degrees of remoteness upon the distinctness 

 of their color and outline, which is known to artists as " aerial perspective." 

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

 atmosphere, as is particularly known to every one who has visited warmer cli- 

 mates; where the extreme clearness of the air sometimes brings into an appa- 

 rently near proximity, a hill that rises some miles beyond a neighboring ridge 

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

 mate of the distance of the remote hill), whilst a slight haziness carries its ap- 

 parent distance to three or four times the reality. 



890. Our estimate of the size of an object is partly dependent upon the visual 

 angle under which we see it, and partly upon our estimate of its distance. The 

 " visual angle," formed by imaginary lines drawn from the eye (Fig. 216, A) to 

 the extreme points, B c, of the object, is the measure of the size of its image 

 upon the retina ; and it is obvious that, if two objects, B c, D E, the former 

 being twice the length of the latter, be placed at the same distance, the visual 

 angle B A c being twice as great as the angle DAE, the image of B c upon the 

 retina will be twice as long as that of D E, and the mind will estimate their 



