"APPRECIATION OF SOUNDS :—TRANSMISSION 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) ; ‘dona, 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 may be made evident 
by a very simple experiment. Place 
acoin 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 ¢, the coin will 
gain become visible, although neither its own place nor that 
| of the observer has undergone any change. The reason of this 
