426 PHYSIOLOGY CHAP. 



from the centre of the fovea outwards, to which he referred 

 certain optical illusions, since a figure cannot be seen simultane- 

 ously as a whole without a portion of it falling on a retinal region 

 in which more irradiation occurs. A. Lehmann considered that 

 irradiation is a direct consequence of contrast in brightness, and 

 noted that when this contrast disappears the illusion disappears 

 also. He therefore considers irradiation not only as one cause, 

 but as the sole cause of the illusions. He admits, however, that 

 in illusions of the Miiller-Lyer type and others the illusory effect 

 is too great to be attributable entirely to irradiation, and con- 

 sequently psychological factors cannot be excluded. Botti, too, 

 in a recent memoir, Optico-geometrical Illusions (1909) which 

 summarises the work he carried out under Kiesow's direction- 

 considers irradiation to be one of the factors that comes into play 

 in the genesis of illusions, but does not regard it as the sole 

 determinant. He concludes that illusions are true psychical 

 " facts " of perception. Not only do we localise the illusory pheno- 

 menon in the object perceived, but we are further capable of 

 recognising it and comparing it with other previously known 

 objects. 



Another phenomenon which in our opinion proves the 

 psychical origin of illusions is the power especially developed in 

 painters of producing a complete and complicated representation 

 of a solid form on paper, with a few rough lines in pencil. This 

 faculty was peculiarly well developed in Leonardo da Vinci, who 

 was able in accidental splotches on the rough wall of a building 

 to see figures of men or animals in different picturesque attitudes 

 according to his fancy. 



The transition from these forms of optical illusion, which we 

 may term const r n rt i r,*, to true visual hallucinations is easy. 

 "Phantasms or hallucinations are" -in the masterly definition 

 of Joh. Miiller "sensory perceptions that depend on internal 

 causes apart from any external exciting objects." 



True visual hallucinations are not those evanescent repre- 

 sentations of figures without light or colour which every one can 

 evoke by imagination on closing his eyes ; they are actual 

 visions of coloured and luminous images produced by central 

 excitation, and perfectly comparable with the perceptions of real 

 objects present in the visual field which excite the retina. 



They are fairly frequent in febrile diseases, in inflammatory 

 states of the brain, in the delirium that accompanies mental 

 diseases, and in certain forms of intoxication. But in conditions 

 of more or less perfect health also they may occur in the half- 

 waking state, in the dreams that accompany sleep, and sometimes 

 even when awake. Blind people too, whose mentality is normal, 

 have been known after extirpation of an eye to suffer from genuine 

 visual hallucinations. 



