OPTICS. 
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OPTICS. 
The subjects treated of in the science of Optics! are Light and 
~ Vision. We learned in the chapter on Acoustics, that the sensations of 
Sound are produced by vibrations in sounding bodies being communi- 
cated to the air, and by it transmitted to the nerves of the ears. It is now 
believed that both Light and Heat consist of a vibratory motion of the 
particles of hght-giving and hot bodies, and that they are transmitted in 
a manner exactly similar to the transmission of sound. The medium, 
however, cannot be the air, since light and heat pass where there is no 
air ; philosophers have accordingly come to the conclusion that all space 
is occupied by an infinitely elastic substance or fluid called ether. The 
vibrations of the particles of a luminous body are communicated to the 
ether ; the pulses transmitted through it enter the eye, and strike upon 
the retina, and, being thence conveyed to the brain, produce the sensation 
of Sight. These vibrations pass from a luminous body in every direction, 
and for this reason Light is said to be composed of rays, from Latin 
radius, the spoke of a wheel, and these rays are said to be divergent. 
Light travels at the rate of about 194,000 miles in a second. 
When light falls on the surfaces of bodies, some or all of the rays are 
reflected, or thrown back by the surface ; or, some or all of the rays are 
transmitted, or pass through the body, according to the nature of the 
body, and the manner in which the light falls upon it. Rays which are 
transmitted from one substance into another, are bent out of the straight 
Jine, and are said to be refracted. The following lessons on Optics consist 
of a short statement of the Laws of Reflection and Refraction. 
Refiection. 
Let AB (fig. 24) be a reflecting surface, as a mirror, with a ray of 
light falling on it in the direction of ed; and let cd be a line perpendi- 
cular to AB: then, by the first law of reflection, 
the ray ed,will be reflected from AB in the 
direction db, so that ed, dc, and db shall all be in 
the same plane; and by the second, the angle 
cdb is equal to the angle cde. A ray falling on 
a surface, as ed, is called the incident? ray, and 
d is called the point of incidence; cd, the per- 
pendicular at the point of incidence, is called 
the normal; and db, the reflected ray. The first law of reflection may 

1 From Greek optikos, relating to sight. 2 From Latin incido, to fall upon, 
