THE EYE, AND THE PHENOMENA OF VISION. 357 



an erroneous application of the experience which in ordi- 

 nary cases supphcs true and accurate conclusions. 



Thus, to most persons a conflagration at night, however 

 misjudge' the distant, appears as if very near. The explanation of this mis- 

 distance of a ^ j.g ig ag follows: — LUAit radiating from a center rapidly 

 fire in the ° ° . , • /. 



night f weakens as the distance from the center increases, being, lor 



instance, only one fourth part as intense at double the dis- 

 tance. The eye learns to make these allowances, and by the clearness and 

 intensity of the light proceeding from the object, judges with considerable ac- 

 curacy of the comparative distance. But a lire at night appears uncommonly 

 brilliant, and therefore seems near. 



The evening-star rising over a hill-top, appears as if situated directly over 

 the top of the eminence. The reason of this also is, that in judging we make 

 brightness and clearness to depend on contiguity, as it ordinarily does; and 

 as the star is bright, we unconsciously think it near us. 



In consequence of terrestrial objects being placed in close 

 and moon ap- comparison, the sun and moon appear larger at their rising 



pear larger ^^^^ setting than at any other time. This illusion is wholly a 

 when nsing and o .' .' 



setting than at mental one, since the organs of vision do not present to us a 

 other umeb? larger image of the sun or moon in the horizon than when in 

 the zenith, or overhead. 



,^^ ^ ,, The moon, although a sphere, appears to be a flat surface, 



Why does the i ^ f t if > 



moon, a sphere, since it is SO remote that we are unable to distinguish any 



appear hke a difierence between the lonGrth of the ravs reflected from the 

 flat surface i ^ - 



circumference, and those reflected from the center. 



Thus the says A D and C D, Fig. 290, appear to be no longer than the ray 

 ^ B D ; but if all the rays seem 



of the same length, the part B 

 I ■u'ill not seem to be nearer to 

 us than A and C ; and there- 

 fore the curve ABC will look 

 like a flat, or horizontal surface. The rays A D and C D are 210,000 miles 

 long. The ray B D is 238,910 miles long. 



What two '^15- I^ order that the eye may see distinctly, 



■emfai foT d1^ the picture formed upon the retina must he 

 tiact vision? illuminated to the right degree, and it must 

 also remain sufficiently long upon the retina to produce a 

 Bcnsation upon the optic nerve. 



The image of an object on the retina may bo illuminated too much or too 

 little to produce a sensible perception of its form. Thus, wo can gain no idea 

 of the form of the sun by viewing it in the clear skj', because the degree of 

 illumination is so great, that the sense of vision is overpowered, just as sounds 

 arc sometimes so intense as to bo deafening. That it is the intense splendor 

 alono which prevents a distinct perception of the sun's figure, is rendered 



