_Illusions of Interrupted Extent._--Distance and area appear to vary in extent, depending upon whether they are filled or empty or are only partially filled. For example, a series of dots will generally appear longer overall than an equal distance between two points. This may be easily demonstrated by arranging three dots in a straight line on paper, the two intervening spaces being of equal extent, say about one or two inches long. If in one of the spaces a series of a dozen dots is placed, this space will appear longer than the empty space. However, if only one dot is placed in the middle of one of the empty spaces, this space now is likely to appear of less extent than the empty space. (See Fig. 7.) A specific example of this type of illusion is shown in Fig. 6. The filled or divided space generally appears greater than the empty or undivided space, but certain qualifications of this statement are necessary. In _a_ the divided space unquestionably appears greater than the empty space. Apparently the filled or empty space is more important than the amount of light which is received from the clear spaces, for a black line on white paper appears longer than a white space between two points separated a distance equal to the length of the black line. Furthermore, apparently the spacing which is the most obtrusive is most influential in causing the divided space to appear greater for _a_ than for _b_. The illusion still persists in _c_. [Illustration: Fig. 6.--The divided or filled space on the left appears longer than the equal space on the right.] An idea of the magnitude may be gained from certain experiments by Aubert. He used a figure similar to _a_ Fig. 6 containing a total of five short lines. Four of them were equally spaced over a distance of 100 mm. corresponding to the left half of _a_, Fig. 6. The remaining line was placed at the extreme right and defined the limit of an empty space also 100 mm. long. In all cases, the length of the empty space appeared about ten per cent less than that of the space occupied by the four lines equally spaced. Various experimenters obtain different results, and it seems reasonable that the differences may be accounted for, partially at least, by different degrees of unconscious correction of the illusion. This emphasizes the desirability of using subjects for such experiments who have no knowledge pertaining to the illusion. [Illustration: Fig. 7.--The three lines are of equal length.] [Illustration: Fig. 8.--The distance between the two circles on the left is equal to the distance between the outside edges of the two circles on the right.]