A few facts pertaining to vision will indicate the trend of developments necessary in the presentation of mobile light.  The visual process synthesizes colors and at this point departs widely from the auditory process.  The sensation of white may be due to the synthesis of all the spectral colors in the proportions in which they exist in noon sunlight or it may be due to the synthesis of proper proportions of yellow and blue, of red, green, and blue, of purple and green, and a vast array of other combinations.  A mixture of red and green lights may produce an exact match for a pure yellow.  Thus it is seen that the mixture of lights will cause some difficulty.  For example, the components of a musical chord may be picked out one by one by the trained ear, but if two or more colored lights are mixed they are merged completely and the resultant color is generally quite different from any of the components.  In music of light, the components of color-chords must be kept separated, for if they are intermingled like those of musical chords they are indistinguishable.  Therefore, the elements of harmony in mobile light must be introduced by giving the components different spatial positions.

The visual process is more sluggish than the auditory process; that is, lights must succeed each other less rapidly than musical notes if they are to be distinguished separately.  The ear can follow the most rapid execution of musical passages, but there is a tendency for colors to blend if they follow one another rapidly.  This critical frequency or rate at which successive colors blend decreases with the brightness of the components.  If red and green are alternated at a rate exceeding the critical frequency, a sensation of yellow will result; that is, neither component will be distinguishable and a steady yellow or a yellow of flickering brightness will be seen.  The hues blend at a lower frequency than the brightness components of colors; hence there may be a blend of color which still flickers in brightness.  Many weird results may be obtained by varying the rate of succession of colors.  If this rate is so low that the colors do not tend to merge, they are much enriched by successive contrast.  It is known that juxtaposed colors generally enrich one another and this phenomenon is known as simultaneous contrast.  Successive contrast causes a similar effect of heightened color.

An effect analogous to dynamic contrast in music may be obtained with mobile light by varying the intensity of the light or possibly the area.  Melody may be simply obtained by mere succession of lights.  Tone-quality has an analogy in the variation of the purity of color.  For example, a given spectral hue may be converted into a large family of tints by the addition of various amounts of white light.  Rhythm is as easily applied to light as to music, to poetry, to pattern, or to the dance, but in mobile lights its limitations already have been suggested.  However, it is bound to play an important part in the art of mobile light because rhythmic experiences are much more agreeable than those which are non-rhythmic.  Rhythm abounds everywhere and nothing so stirs mankind from the lowliest savage to the highly cultivated being as rhythmic sequences.