584 THE NERVOUS SYSTEM. 



enough to stimulate a cone, it does not come within the range of 

 vision. 



Distance. It is impossible to judge of the distance of objects 

 except by experience ; thus a child reaches for everything it sees, 

 irrespective of the distance from it the objects may be ; and persons 

 who, having been blind from birth, are in maturer years endowed 

 with sight by operation, for instance bear testimony that every- 

 thing seems to be immediately in front of them. If, however, the 

 size of an object is known, then the size of its image on the retina 

 determines our estimate of its distance. Conversely, if we know 

 the distance of an object, then the image which that object produces 

 on the retina is the determining factor in our judgment of its 

 actual size. If, therefore, our judgment of the distance of an 

 object from us is erroneous, so will be our judgment of its size, 

 and vice versa. If, for instance, a ship is seen through a fog, we 

 suppose that, being indistinctly seen, it is at a considerable dis- 

 tance from us, although, as a matter of fact, it may be quite near, 

 and making an image of considerable size upon the retina, we judge 

 the ship to be larger than it actually is. It is a well-known fact 

 that the moon seems larger to an observer when near the horizon 

 than when near the zenith, although, as a matter of fact, it is nearer 

 by about 4000 miles, half the diameter of the earth, when in the 

 zenith, and should therefore a priori seem larger, but when it is 

 near the horizon we have terrestrial objects to compare it with, 

 while when in the zenith there is nothing with which to com- 

 pare it. 



The correctness of this explanation has been questioned by a 

 critic, who says : " The moon looks large when rising on Salisbury 

 Plain, on which there is no conspicuous terrestrial object with 

 which it can be compared, and looks small at its zenith when close 

 to the vane of the spire of Salisbury Cathedral, the size and dis- 

 tance of which are well known. It is probably an affair of re- 

 fraction." 



As this illusion is almost universal, and one of great interest, 

 the writer deemed it worth while to obtain the views of several 

 well-known physicists. Among those written to was Prof. Spice, 

 of Cooper Union, New York. In commenting on the criticism 

 above quoted, he says : " Though it is true that there is no i con- 

 spicuous ' object in sight, still there is at least the Plain and this 

 would give one an idea of distance. With reference to the prox- 

 imity to the vane on the cathedral spire, I would suggest that 

 unless an observer were on the spire the vane itself would (of 

 necessity) look small, and therefore would not tend to make the 

 moon look large. As to the idea of refraction increasing the ap- 

 parent size : The refraction at the horizon is about 34' + ; the 

 apparent diameter of the moon is about 31' + ; so if the moon 



