768 OF THE ORGANS OF THE SENSES, AND THEIR FUNCTIONS. 



625. In regard to remote objects, our judgment of Distance 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 dif- 

 ferent 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 par- 

 ticularly known to every one who has visited warmer climates ; where the 

 extreme clearness of the air sometimes brings into apparently 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 estimate of the distance 

 of the remote hill), whilst a slight haziness carries its apparent distance to 

 three or four times the reality. Additional means of judging of the distance 

 of remote objects pointed out by Aubert, 1 are found in their parallax or ap- 

 parent change of place, on the alteration of our own position with regard to 

 them, and in the rate of movement, both absolute and relative, of the objects 

 themselves, if they happen to be in motion. 



626. Our estimate of the Size of a remote object is partly dependent upon 

 the visual angle under which we see it, and partly upon our estimate of its 

 distance.' 2 The " visual angle," formed by imaginary lines drawn from the 

 eye (Fig. 274, A) to the extreme points, B, c, of the object, is the measure of 

 the dimension of its image upon the retina ; and it is obvious that, if two ob- 

 jects, B c, D E, the former twice the length of the latter, be placed at the 

 same distance, the visual angle BAG 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 relative sizes accordingly. But if the distance of 

 the object D E from the eye be diminished to one-half, so that it is brought 

 into the position F G, its visual angle, and consequently the size of its image 

 on the retina, will now be equal to that of B c ; and the estimate we form of 

 the relative sizes of the two, will entirely depend upon the idea we entertain 

 of their relative distances. Hence any circumstance which modifies that 

 idea, produces a corresponding difference in our estimate of their size; so 

 that the apparent size of an object, seen under a given visual angle, may be 

 estimated as larger or smaller than the reality, according as we suppose it to 

 be more or less distant than it really is. Of this we have a familiar instance 

 in the fact, that if we meet a child whilst we are walking across a common 

 in a fog (the flatness of the ground not giving us much power of estimating 

 the intervening space), it appears to have the stature of a man, and a man 

 seems like a giant; for the indistinctness of outline causes the mind to con- 

 ceive of the figures as at a greater distance than they really are, and their 

 apparent dimensions are augmented in like proportion. For if the object 

 F (; (Fig. 274) be mentally carried back to the distance of D E, being still seen 

 under the visual angle FAG (or B A c), it will appear to possess the length 

 B c instead of n E. On the other hand, if the object B c were to be iiiciitttlh/ 

 brought forward into a position K L, its apparent size being still determined 

 by its visual angle, it will seem to be reduced to the length F G. That our 

 estimate of the Size of near objects, however, depends upon a more direct 



1 Physiologic der Netzhaut, 18B4, p. 17. 



'* When objects are so remote that we have no means of even approximately esti- 

 mating their <l!s/ance, we have no measure whatever of their fsi-e. Thus, the Sun and 

 the Moon are of nearly the same apparent size to us, though one is about four hundred 

 times the distance of the other; and we may cover either disk with a sixpence held 

 near the eye, so as to be seen under the same visual angle ; but we do not possess the 

 least power of estimating the actual sizes of those objects, save by a calculation based 

 on a knowledge of their relative distances. 



