Some objects we know by sight only, and other objects we know by sound only, and other objects we observe neither by light nor by sound but by touch or smell or otherwise.

The velocity of light varies according to its medium and so does that of sound.

Light moves in curved paths under certain conditions and so does sound.

Both light and sound are waves of disturbance in the physical characters of events; and (as has been stated above, p.

188) the actual course of the light is of no more importance for perception than is the actual course of the sound.