ll
end is directed forwardly, and is reflected from the sides, and its
resonance also enables the vibrations to carry farther than without the
use of the solid part of the instrument.

The ear trumpet is an illustration of a sound-collecting device, the
waves being brought together by reflection.

THE STETHOSCOPE.--This is an instrument used by physicians, and it is so
delicate that the movements of the organs of the body can be heard with
great distinctness. It merely collects the vibrations, and transmits
them to the ears by the small tubes which are connected with the
collecting bell.

THE VITASCOPE.--Numerous instruments have been devised to determine
the rate of vibration of different materials and structures, the most
important being the _vitascope_, which has a revolvable cylinder,
blackened with soot, and this being rotated at a certain speed, the
stylus, which is attached to the vibrating body, in contact with the
cylinder, will show the number per second, as well as the particular
character of each oscillation.

THE PHONAUTOGRAPH.--This instrument is used to register the vibration of
wind instruments, as well as the human voice, and the particular forms
of the vibrations are traced on a cylinder, the tracing stylus being
attached to a thin vibrating membrane which is affected by the voice or
instrument.

THE PHONOGRAPH.--This instrument is the outgrowth of the stylus forms of
the apparatus described, but in this case the stylus, or needle, is
fixed to a metallic diaphragm, and its point makes an impression on
suitable material placed on the outside of a revolvable cylinder or
disc.

Light.-Light is the agent which excites the sensation of vision in the
eye. Various theories have been advanced by scientists to account for
the phenomenon, and the two most noted views are the _corpuscular_,
promulgated by Sir Isaac Newton, and the _undulatory_, enunciated by
Huygens and Euler.

The _corpuscular_ theory conceives that light is a substance of
exceedingly light particles which are shot forth with immense velocity.
The _undulatory_ theory, now generally accepted, maintains that light is
carried by vibrations in ether. Ether is a subtle elastic medium which
fills all space.

_Luminous_ bodies are those like the sun, which emit light. Rays may
_diverge_, that is, spread out; _converge_, or point toward each other;
or they may be _parallel_ with each other.

VELOCITY OF LIGHT.--Light moves at the rate of ab