[79] It seems to also flow backwards in certain hypnotic hallucinations. Suggest to a 'Subject' in the hypnotic trance that a sheet of paper has a red cross upon it, then pretend to remove the imaginary cross, whilst you tell the Subject to look fixedly at a dot upon the paper, and he will presently tell you that he sees a 'bluish-green' cross. The genuineness of the result has been doubted, but there seems no good reason for rejecting M. Binet's account (Le Magnetisme Animal, 1887, p. 188). M. Binet, following M. Parinaud, and on the faith of a certain experiment, at one time believed, the optical brain-centres and not the retina to be the seat of ordinary negative after-images. The experiment is this: Look fixedly, with one eye open, at a colored spot on a white background. Then close that eye and look fixedly with the _other_ eye at a plain surface. A negative after-image of the colored spot will presently appear. (Psychologie du Raisonnement, 1886, p. 45.) But Mr. Delabarre has proved (American Journal of Psychology, ii. 326) that this after-image is due, not to a higher cerebral process, but to the fact that the retinal process in the _closed_ eye affects consciousness at certain moments, and that its object is then projected into the field seen by the eye which is open. M. Binet informs me that he is converted by the proofs given by Mr. Delabarre. The fact remains, however, that the negative after-images of Herr Meyer, M. Féré, and the hypnotic subjects, form an exception to all that we know of nerve-currents, if they are due to a refluent centrifugal current to the retina. It may be that they will hereafter be explained in some other way. Meanwhile we can only write them down as a paradox. Sig. Sergi's theory that there is _always_ a refluent wave in perception hardly merits serious consideration (Psychologie Physiologique, pp. 99, 189). Sergi's theory has recently been reaffirmed with almost incredible crudity by Lombroso and Ottolenghi in the Revue Philosophique, xxix. 70 (Jan. 1890). [80] Lotze, Med. Psych. p. 509. [81] See an important article by Binet in the Revue Philosophique, xxvi. 481 (1888); also Dufour, in Revue Méd. de la Suisse Romande, 1889. No. 8, cited in the Neurologisches Centralblatt, 1890. p. 48. [82] Die Willenshandlung (1888), pp. 129-40. CHAPTER XIX. THE PERCEPTION OF 'THINGS.' PERCEPTION AND SENSATION COMPARED.