APPEECIATION OF SOUNDS I TKANSMISSION OF LIGHT. 413 



relative intensity of the impressions received by the two 

 ears ; though we may form some notion of it by either 

 singly ( 520). Of the distance we judge by the intensity of 

 the sound, comparing it with that which we know the same 

 body to produce when nearer to us. The Ear may be deceived 

 in this respect as well as the eye ( 566) ; thus the effect of 

 a full band at a distance may be given by the subdued tones 

 of a concealed orchestra close to us ; and the Ventriloquist 

 produces his deceptions by imitating, as closely as possible, 

 not the sounds themselves, but the manner in which they 

 would strike the ear. 



Sense of Sight. 



526. By the faculty of Sight we are made acquainted, in. 

 the first place, with the presence of light ; and by the medium 

 of that agent we take cognizance of the forms of surrounding 

 bodies, their colours, dimensions, and positions. It is desira- 

 ble that a short account should be here given of the laws of 

 the transmission of light; since, without the knowledge of 

 them, the beautiful action of the Eye cannot be understood. 



527. The rays of light uniformly travel in straight lines, 

 so long as they traverse the same medium (air, water, or 

 glass, for instance), without obstruction. When issuing from 

 a single luminous point into space, they diverge or separate, 

 in such a manner as to cover a larger and larger surface as 

 they proceed ; and the intensity of the light diminishes in the 

 same proportion. But when the rays pass from one medium to 

 another either more or less dense, 



they are bent out of their straight 

 course, or refracted; unless they 

 should happen to pass from the one 

 to the other in a direction perpendi- 

 cular to the plane which separates 

 them. This maybe made evident 

 by a very simple experiment. Place 

 a com or any heavy body (a, fig. 205) 

 at the bottom of a basin, and then 

 retreat from it until the coin is hidden by the edge of the basin ; 

 if water be then poured-in, up to the level c, the coin will 

 again become visible, although neither its own place nor that 

 of the observer has undergone any change. The reason of this 



