SENSE OF VISION. 



889 



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 

 the same 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 (the flatness of the ground not giving us much power 

 of estimating the intervening space) in a fog, 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 conceive 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 G (Fig. 216) 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 D E. On the other hand, if the object B c were to be mentally 

 brought forwards into the position K L, its apparent size being still determined 

 by its visual angle, it will seem to be reduced to the length F G. This has been 

 demonstrated by a very ingenious experiment devised by Prof. Wheatstone. 

 For if two similar pictures placed in his mirror-stereoscope, be made so to change 

 their places in regard to the mirrors (by moving in a horizontal circle of which 

 the middle point between the mirrors is the centre), that the angle of converg- 

 ence of the optic axis is increased, whilst the actual distance of the pictures 

 from the mirrors, and consequently their visual angles, remain the same, their 

 apparent size is progressively and most remarkably diminished ; the mind being 

 accustomed to interpret increase of the angle of convergence as a proof of dimi- 

 nution of distance, and being thus impressed by the change as if the pictures 

 had really advanced to K L ; and as they are still seen under the angle BAG 

 (or F A G), instead of under the angle K A L, their dimensions are reduced to 

 the mind's eye from B C to F G. A very simple and beautiful illustration of the 

 same principle is furnished by the ordinary stereoscope, when two pairs of 

 figures (such as those given in Fig. 215) are employed, the effect of one of which 



