Characteristics of { intensity--area, amplitude, density, distance. Musical Sounds: { pitch--scales; major, tempered, triads, _N_ = { _V_/_L_ quality--fundamental and overtones. Sympathetic Vibrations--resonance, interference, beats, discord. Musical Instruments--string, air column, membrane or plate. Laws of; (a) vibrating strings (3), (b) vibrating air columns (2). CHAPTER XVI LIGHT (1) LIGHT, ITS RECTILINEAR PROPAGATION, SHADOWS =352. A Comparison of Sound and Light.=--Light from the standpoint of physics is considered much as is _sound_, as a _mode_ of _motion_; one affecting the ear, the other producing the result called _vision_. There are other differences also worth considering. (a) While sound travels as vibrations of some _material_ medium, light travels only as vibrations of the _ether_; solids, liquids, and gases act so as to hinder rather than to assist in its movement. That is, light travels best in a vacuum or in a space devoid of ordinary matter. (b) The _speed_ of light is so great that at ordinary distances on the earth its motion is practically instantaneous. Experiments have shown that its speed is about 186,000 miles to 300,000 kilometers a second. =353. Luminous and Illuminated Bodies.=--If we consider the objects within a room, some of them, as books and furniture, would be invisible if all light from external sources were excluded. On the other hand, some other objects, such as a lighted lamp, a burning coal, or a red hot iron, would be seen if no outside light were present. Such bodies are said to be luminous. Most luminous bodies are hot and become non-luminous on cooling. There are, however, some bodies that are luminous at ordinary room temperatures, as the firefly and some phosphorescent paints. When light emitted by a luminous body strikes an object, a portion of it is always _reflected_. It is this reflected light that makes the illuminated object _visible_. If the object is a sheet of glass, some of the light is _transmitted_. If a substance is so clear that objects can be seen through it, the substance is _transparent_, but if objects cannot be seen through it, the substance is said to be _translucent_. Objects transmitting no light are _opaque_. Some of the light falling upon a body is neither reflected nor transmitted, but is _absorbed_ and tends to warm the body. The light falling upon a body is therefore either _reflected_, _transmitted_, or _absorbed_. Thus Fig. 345 represents light coming from _S_ to a piece of glass _GL_. A portion of the light represented by _R_ is reflected. Another part _A_ is absorbed and disappears, while still another part _T_ is transmitted and passes on. [Illustration: FIG. 345.--The light is transmitted (_T_), reflected (_R_), or absorbed (_A_).]