We could go further into the psychology of light and color and conjecture regarding effects directly attributable to color, such as excitement, depression, and tranquillity. In so doing we would be led far astray from illusions in the sense of the term as used here. Although this term as used here is still somewhat restricted, it is broader in scope than in its usual applications. However, it is not broad enough to lead far into the many devious highways and byways of light and color. If we did make these excursions we would find associations almost universally answering the questions. The question would arise as to innate powers of colors and we would find ourselves wondering if all these powers were acquired (through associations) and whether or not some were innate. And after many interesting views of the intricate subject we would likely conclude that the question of the innateness of some of the powers of color must be left unanswered. As an example let us take the case of the restfulness or depression due to blue. We note that the blue sky is quite serene or tranquil and we find that the delicate sensibilities of poets verify this impression. This association could account for the impression or feeling of tranquillity associated with blue. On proceeding further, we would find nature's solitudes often tinged with the blue skylight, for these solitudes are usually in the shade. Thus their restfulness or even depressiveness may be accounted for--partially at least. These brief glimpses are presented in order that they may suggest to the reader another trend of thought when certain illusions of light and color are held up for analysis. Besides these our individual experiences which have molded our likes and dislikes must be taken into account. This phase of light and color has been treated elsewhere.[6] A very unusual kind of optical illusion is illustrated by the phenomenon of the apparent ending of a searchlight beam which has attracted much attention in connection with the powerful searchlights used for locating aeroplanes (Fig. 77). For years the apparent ending has more or less carelessly been attributed to the diminution of the density of atmospheric fog or haze, but recently Karrer[13] has suggested what appears to be the correct explanation. When the beam of light from a powerful searchlight is directed into space, its path is visible owing to the scattering of some of the light by dust and moisture particles and the molecules of the air itself. While obviously the beam itself must go on indefinitely, its luminous path appears to end abruptly at no very great distance from the source. This is true whether the beam is photographed or viewed with the naked eye. [Illustration: Fig. 77.--Apparent ending of a searchlight beam.]