1 1 46 VISION. 



vision has been explained by Einthoven 1 differently. If a series of 

 alternate red and blue strips are looked at with one eye and an eccentric 

 pupil, one of the colours will appear in relief. The raised colour depends 

 on the nature of the eccentricity and on the direction from which the 

 strips are observed. Einthoven points out that the red and blue will 

 be displaced laterally on the retina giving rise to dark and light borders 

 which appear to separate the strips, and have the same effect as 

 shadows. The colour relief, depending on the distribution of the light 

 and shade, will consequently vary with the direction from which the 

 colours are regarded. 



Another appearance which may be referred to the same factors as 

 colour stereoscopy, is the apparent movement of different parts of a 

 coloured (especially red and blue) pattern. Such a pattern, when moved, 

 especially in peripheral vision, gives a nickering appearance, which in 

 some people produces dizziness and even vomiting. The apparent differ- 

 ence of movement is naturally explained by the difference of focus for 

 different colours, and greater excursion of the more peripheral focus on 

 movement (Schapringer). 2 



Visual perception of movement. — A moving object must have a 

 certain velocity to be seen as moving. Aubert 3 has found this velocity 

 in direct vision to be between 1' and 2' in the second. At about 1', 

 movement is only seen after some seconds ; while with slower move- 

 ment, namely, about 40" per second, the object appears to be at rest. 

 The retinal distance corresponding to 1' being about 4 - 36 (t, the thres- 

 hold of movement for the fovea corresponds fairly closely with the 

 limit of visual acuity. It was found difficult to investigate the just 

 perceptible difference of velocity, but for small velocities it was 

 found to be about the same as the threshold, namely, 1'. For 

 peripheral vision, Aubert found that the angular velocity had to be 

 increased for movement to be seen, but the decrease of sensibility 

 for movement from centre to periphery was proportionally very much 

 less than the decrease in visual acuity. On the other hand, there is 

 evidence that the peripheral retina is especially sensitive to move- 

 ment. It is a familiar experience in perimetric observations that at 

 the extreme periphery of the retina an object is not seen at all till it 

 moves, and that this movement may be seen by an individual who is 

 unable to describe in any way the object which is moving. According 

 to Exner, 4 there is a tendency to see any kind of change (change of 

 intensity) in the periphery as movement. It is generally supposed that 

 seeing a movement is due to a more or less complicated mental process ; 

 that a body is seen at a at the moment t, at a' at the moment t', and 

 at a" at the moment t", and that it is inferred that the body has moved 

 from a to a". Exner 5 supposes that there is in addition direct sensibility 

 to movement, immediate sensation of movement. Animals often fail to 

 see an object till it moves, even when it is quite close ; and it is to be 

 expected that seeing movement in peripheral vision should be one of 

 the most primitive properties of the visual apparatus. 



Certain after-image phenomena, which are consistent with direct 



1 Brain, London, 1893, vol. xvi. p. 191. 



2 Ztschr. f. Psychol. u. Physiol, d. Sinnesorg., Hamburg u. Leipzig, 1893, Bd. v. S. 385. 



3 Arch.f. d. ges. Physiol., Bonn, 1886, Bd. xxxix. S. 347 ; and 1887, Bd. xl. S. 459. 



4 Arch./. Ophth., 1886, Bd. xxxii. Abth. 1, S. 233. 



6 Sitzungsb. d. 1c. Akad. d. Wissensch., Wien, 1875, Bd. lxxii. Abth. 3, S. 156. 



