IN POPULAR PHYSICS. 47 



44:. Why does a bright light, in the night, seem so much 

 nearer than it is ? 



We judge of the distance of an object by its magnitude, by 

 its distinctness of outline, and by the size, etc., of intervening 

 objects with which we compare it. In the night, the Ixrightness 

 of a light confuses us by its vividness, seeming to be near at 

 hand. Moreover, we can not see the neighboring objects, whose 

 distance we know or could estimate in the daylight. Our error 

 is therefore one of judgment. 



45. What color predominates in artificial lights ? 



Yellow. 



46. Why are we not sensible of darkness when we 

 wink ? 



Because the wink does not last so long as the impression of 

 the light received just before the wink. 



47. Under ivhat condition do the eyes of a portrait 

 seem to follow a spectator to all parts of a room ? 



This is noticed only in a full-face portrait. In that case the 

 spectator, when he goes to either side, fails to see the side of 

 the eyeballs, and hence the effect is that of looking directly into 

 the eye. "A rifleman, portrayed as if taking aim directly in 

 front of the picture, appears to every observer to be pointing 

 at him specially." 



48. Why do the two parallel tracks of a railroad ap- 

 pear to approach in the distance ? 



The visual angle subtended by the distance between two op- 

 posite points on the tracks becomes less as the distance of the 

 observer increases. 



49. W7iy does a fog apparently magnify objects ? 



It is not the refraction of the rays of light, as is commonly 

 supposed, which makes an object seem larger when seen through 

 a mist. It really appears to us in its proper size. The mist, 

 however, dims the color and the outline, giving it the indis- 

 tinctness belonging to a mile in distance, while it has the mag- 

 nitude of half a mile. Dr. "Wayland relates that, as he was 



