Where interference of greenish blue occurs, red is found, etc. Many phenomena are due to interference, such as (A) the color of thin films of oil on water, where the portions of light reflected from the two surfaces of the oil film interfere resulting in the production of color; (B) the color of soap bubbles. When first formed, soap-bubble films are not thin enough to show interference well, but as the bubbles increase in size or become thinner on standing, the conditions for interference are reached and, as the film becomes thinner, a regular succession of colors is noticed. =413. Differences Between Light and Sound.=--Among the important differences between light and sound that have been considered are the following: the former are (a) _waves_ in the ether, (b) _of very short wave length_, and (c) their _motion is in straight lines_. Another difference (d) is in _the mode of vibration_. Sound waves are _longitudinal, while light waves are transverse_. Light waves consist of vibrations of the ether at right angles to the line of motion. To illustrate the reasoning that has led to this conclusion, suppose a rope to be passed through two vertical gratings. (See Fig. 411, 1.) If the rope be set in _transverse_ vibration by a hand, the waves produced will readily pass through to the gratings _P_ and _Q_ and continue in the part extending beyond _Q_. If, however, _Q_ is at right angles to _P_, no motion will be found beyond _Q_. Now if a stretched coiled spring with longitudinal vibrations should take the place of the rope, it is evident that the crossed position of the two gratings would offer no obstacles to the movement of the vibration. In other words, crossed gratings offer no obstruction to longitudinal vibrations, while they may completely stop transverse vibrations. [Illustration: FIG. 411.--Transverse waves will pass through both gratings in (1) where the openings in the two gratings are at right angles. The waves passing _P_ are stopped by _Q_ (2).] [Illustration: FIG. 412.--Effect of tourmaline crystals on light.]