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.
Fig. 7.—The three lines are of equal length.
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.
As already stated there are apparent exceptions to any simple rule, for, as in the case of dots cited in a preceding paragraph, the illusion depends upon the manner in which the division is made. For example, in [Fig. 7], a and c are as likely to appear shorter than b as equal to it. It has been concluded by certain investigators that when subdivision of a line causes it to appear longer, the parts into which it is divided or some of them are likely to appear shorter than isolated lines of the same length. The reverse of this statement also appears to hold. For example in [Fig. 7], a appears shorter than b and the central part appears lengthened, although the total line appears shortened. This illusion is intensified by leaving the central section blank. A figure of this sort can be readily drawn by the reader by using short straight lines in place of the circles in [Fig. 8]. In this figure the space between the inside edges of the two circles on the left appears larger than the overall distance between the outside edges of the two circles on the right, despite the fact that these distances are equal. It appears that mere intensity of retinal stimulation does not account for these illusions, but rather the figures which we see.