1 1 44 VISION. 



shown in Fig. 414. It has been referred to the erroneous estimation of 

 angles, and to psychological factors. The most satisfactory explana- 

 tion is that our judgment of the length of the line ab is influenced by 

 the idea of other imaginary lines, a'b', a"~b". The illusion is diminished by 

 making the base line ab more obvious, and increased by making the 

 end lines more distinct. This illusion has been investigated quantitat- 

 ively by Hey mans, who found the illusion increased with the acuity 

 of the angle. It increased, but not continuously with increase in length 

 of the end lines, but attained its maximum when the end lines were 

 between 20 and 40 mm. (with a base line of 75 mm.). Heymans 1 

 supposed that this result contradicted the above explanation, but it is 

 to be expected that the distances near the base line would influence 



FIG. 414. Auerbach. 



perception to a much greater extent than those more remote, and that 

 when the two lines at each end became longer than the base line, a new 

 kind of figure would be produced and new factors introduced. 2 



Colour stereoscopy. In a red and blue pattern on a black back- 

 ground one of the two colours may be observed to stand out in 

 relief. The phenomenon occurs for other colours, but is more marked the 

 farther the two colours lie from one another in the spectrum. The first 

 observers of this appearance saw red (or yellow) stand out in relief, but 

 it has been found that to some individuals blue appears nearer, and 

 that there is little difference in the numbers of those who see red or 

 blue in relief respectively, while to others the phenomenon is very 

 indistinct, or is not observable under ordinary conditions. In marked 

 cases (Bonders) the colour may appear to stand out as much as 20 cm. from 

 the background at a distance of 4 metres. The appearance only occurs 

 to a marked extent with binocular vision, but under certain conditions 

 may be observed with one eye, though much less distinctly. It has 

 been found by Einthoven 3 to be to a considerable extent dependent on 

 the eccentricity of the pupil ; individuals with the pupil situated on the 

 temporal side of the optic axis see red in front of blue, while those with 

 nasal pupils see blue in relief. This comes out much more clearly if 

 the pupils are dilated with atropin, and a small artificial pupil placed 

 before each eye. When the artificial pupils are separated from one 

 another, red is in relief, while, as they are approached to one another, 

 blue starts forward. This may also be observed by separating and 

 approaching to one another lenses held before the eyes. Hess made 

 use of this fact to test the movements of the lens described by him 

 (see p. 1035). With convex lenses, situated temporally, blue is seen in 

 front ; when situated nasally, red appears in front. Hess put eserin in 

 one eye, and found, on inclining the head to the same side, thus making 



1 Ztschr.f. Psychol. u. PhysioL d. Sinncsorg., Hamburg u. Leipzig, 1896, Bd. ix. S. 221. 



2 For reference to the chief papers on this illusion, see ibid., 1896, Bd. ix. S. 16. 



3 Arch.f. Ophth., 1885, Bd. xxxi. Abth. 3, S. 211. 



