This effect is perhaps best witnessed on viewing a line spectrum such as that of the mercury arc, focused upon a ground glass. The violet and blue lines are not seen in good focus when the eyes are focused upon the green and yellow lines. Furthermore, the former can be seen in excellent focus at a distance too short for accommodating the eyes to the green and the yellow lines. This experiment shows that the focal length of the optical system of the eye is considerably shorter for the spectral hues of shorter wave-length (violet, blue) than for those of longer wave-length (such as yellow). Narrow slits covered with diffusing glass and illuminated respectively by fairly pure blue, green, yellow, and red lights may be substituted. The effect may be demonstrated by trying to focus fine detail such as print when two adjacent areas are illuminated by blue and red lights respectively. It is also observed when fine detail such as black lines are held close to the eye for colored fringes are seen. This optical defect is responsible for certain visual illusions. An excellent demonstration of chromatic aberration in the eye is found by viewing fine detail through a purple filter. Now if a red filter be superposed on the purple one only the red light is transmitted. Notwithstanding the decrease in illumination or rather of light reaching the eye, measurement shows that finer detail can be discriminated than in the first case. A similar result is found on superposing a blue filter upon the purple one. _Retiring and Advancing Colors._--For years the artist and the decorator have felt that certain colors seem to advance nearer than others or that the latter seem to retire more than the former. The author[5] obtained actual measurements of this phenomenon, but the evidence also indicated that the effects were not the same for all persons. The phenomenon is very noticeable in the case of the image of a colored lantern-slide projected upon a screen and is readily observed when the image consists of letters of various colors. In the case of red and green letters, for example, the former appear (to most persons) to be considerably nearer the observer than the green letters. It has appeared to the writer that the illusion is apparent even for white letters upon a dark background. In general, the colors whose dominant hues are of the shorter wave-lengths (violet, blue, blue-green, green) are retiring and those whose dominant hues are of the longer wave-lengths (yellow, orange, red) are advancing. [Illustration: Fig. 68.--For demonstrating retiring and advancing colors.]