The explanation which the theory of Hering gives of afterimages and of simultaneous contrast are not explanations at all, but merely translations of the facts into the language of his theory. My theory is able to deal with them more satisfactorily; when red light, say, has been acting upon the retina for some time, many of the photo-chemical molecules have lost that one of their constituents which is the exciter of the red sensation; but in this mutilated condition they are exceedingly unstable, and their other two constituents (the exciters of the sensations of blue and of green) are gradually set free; the effect of this is that, while the eyes are still open a blue-green sensation is added to the red sensation with the result of making it gradually fade out into white, and, if the eyes are closed, the cause of the bluegreen sensation persists until all the molecules affected are totally decomposed. Thus the actual course of the sensation produced by looking at a red object, its gradual fading out, in case of careful fixation, and the appearance of the complementary color if the illumination is diminished or if the eyes are closed, is exactly what the original assumption of a partial decomposition of molecules would require us to predict. The well-known extreme rapidity of the circulation in the retina would make it impossible that the partly decomposed molecules, just referred to should remain within the boundaries of the portion of the retina in which they are first produced; and their completed decomposition after they have past beyond these boundaries is the cause of the complementary color-sensation which we call simultaneous contrast. The spreading of the actual color which succeeds it would then be accounted for, as Helmholtz suggests, by a diffusion of the colored light in the various media of the eye. No effort has hitherto been made to explain a very remarkable feature in the structure of the retina,-the fact that the retinal elements are of two different kinds, which we distinguish as rods and cones. But this structure becomes quite what one might expect, if we suppose that the rods contain the undeveloped molecules which give us the sensation of grey only, while the cones contain the color molecules, which cause sensations of grey and of color both. The distribution of the rods and cones corresponds exactly with the distribution of sensitiveness to just perceptible light and color excitations as determined by the very careful experiments of Eugen Fick1 1 Studien über Licht und Farbenempfindung « Pflüger's Archiv," Bd. XLIV., s. 441, 1888. Two other theories of light-sensation have been proposed besides the one which I have here outlined, either one of which meets the requirements of a possible theory far better than those of Hering or of Helmholtz; they are those of Göller1 and Donders 2 The former is a physical theory. That of Donders is a chemical theory, and very similar to the one which I here propose. Every chemical theory supposes a tearing down of highly complex molecules; Donder's theory supposes in addition that the tearing down in question can take place in two successive stages. But Donders' theory is necessarily a four-color theory; and Donders himself, although the experiments of König above referred to had not at that time been made, was so strongly convinced of the necessity of the three-color theory for the explanation of some of the facts of color-vision that he supplemented his four-process theory in the retina with a three-process theory in the higher centres. The desirableness, therefore, of devising a partial decomposition of molecules of such a nature that the fundamental color-processes assumed can be three in number instead of four is apparent. But the theory of Donders is open to a still graver objection. The molecules assumed by him must, in order to be capable of four different semi-dissociations, consist of at least eight different atoms or groups of atoms. The red-green dissociations and the yellow-blue dissociations we may then represent symbolically by these two diagrams respectively: But it will be observed that the two completed dissociations end by having set free different combinations; in the one case 1 is combined with 2 and in the other case 1 is combined with 8, etc. If, now, the partial dissociations are so unlike as to cause sensations of yellow and blue (or of red and green) it is not probable that completed dissociations which end in setting free different chemical combinations should produce the same sensa 1 Die Analyse der Lichtwellen durch das Auge. "Du Bois Reymond's Archiv," 1889. 2 Noch einmal die Farben-systeme. B.S. 30, (1), 1884. "Gräfe's Archiv für Ophthalmologie," tion, grey. The difficulty introduced by Donders' theory is therefore (as in the case of Hering's theory) as great as the difficulty sought to be removed. It is the desire to secure the advantages of a partial dissociation theory, without the disadvantages of the theory of Donders that has led me to devise a partial dissociation of molecules of a different kind. The theory will be found more explicitly set forth in the next number of the Zeitschrift für Psychologie. [The notes of the discussion on Mrs. LADD FRANKLIN'S paper have unfortu nately been lost. I can only venture to say from recollection that the rela tion between her theory and that of Donders was the subject mainly debated.— H. SIDGWICK]. WEDNESDAY, August 3rd, 1892. PROFESSOR EBBINGHAUS IN THE CHAIR. Professor VICTOR HORSLEY in answering the criticisms which had been passed on his communication of the previous day said: With regard to the statements of Professor Ferrier and the latter's quotation from Dr. Mills he desired to say that as he had demonstrated it at Philadelphia, the methods employed by these neurologists, though of course perfectly adequate for revealing the presence or absence of marked analgesia or anesthesia, were yet examples of maximum excitation, and as such were not adequate for demonstrating the presence of a slight degree of tactile anæsthesia; and further, he desired to reiterate the necessity of employing Westphal's method, viz., causing the blindfolded patient to indicate with the opposite hand the point lightly touched by the observer. Great confusion seemed to him also to rest in the minds of many concerning the representation of tactile æsthesia found by Professor Ferrier to be situated in the hippocampal gyrus and by Professor Schäfer and himself also in the gyrus fornicatus. It must be perfectly patent to many that whereas it is likely that certain degrees and extent of sensation may well be represented in the limbic lobe it is also not only conceivable, but, in view of the sensory-motor evolution of the central nervous system, necessary, that there should also be represention, in the so-called motor region, of such sensory impressions as may be necessary to the perfect coordination of deliberate movement. With regard to Dr. Waller's statement that the exciting of the central end of the anterior root and recording by the electrical method of Professor Gotch and himself threw no light upon and gave no justification to the views of Professors James and Bastian, and further, added nothing to what had been stated by Bell, he would only refer those interested, to the perusal of the original documents of the authors named. Professor HEYMANS, of Groningen, then read the following paper: Ueber das Verhältniss des Weber'schen Gesetzes zu den Erscheinnungen der Vorstellungshemmung. Eine verläufige Mittheilung. Frühere noch nicht abgeschlossene Versuche, hatten mich gelehrt, dass die Reizschwelle für Druckempfindungen auf dem Handrücken beträchtlich steigt, wenn gleichzeitig mit dem zu prüfenden Reize, in 4 cm. Entfernung von demselben, ein zweiter, constanter Druckreiz einwirkt ;-dass also eine Druckempfindung, welche an und für sich merklich sein würde, durch das gleichzeitige Gegebensein einer anderen hinreichend starken Druckempfindung unmerklich gemacht (gehemmt) werden kann. Dieses Ergebniss führte mich zur Frage, ob vielleicht in ähnlicher Weise auch eine Unterschiedsempfindung zweier Druckreize, welche an und für sich merklich sein würde, durch das gleichzeitige Gegebensein einer hinreichend starken Druckempfindung unmerklich gemacht werden könne. Zur Beantwortung derselben stellte ich nach der Methode der richtigen und falschen Fälle eine Untersuchung an, deren vorläufiges Ergebniss der aufgestellten Vermuthung vollständig entspricht. Denn es zeigte sich dass, während die Unterschiedsschwelle bei Reizen von 10, 20, 40, 80, 120 und 180 qr., bezw. 5.0, 4.4, 7.2, 11.3, 18.4 und 26.4 qr. betrug, dieselbe sich bei gleichzeitiger Einwirkung eines 4 cm. entfernten, die fünffache Intensität der Vergleichsreize besitzenden Hemmungsreizes bis auf 5.2, 9.4, 13.5, 26.0, 39.2 und 38.5 qr., also um die Betrage von 0.2, 5.0, 6.3, 14.7, 20.8 und 32.2 qr. erhöhte. Aus diesen Zahlen scheint hervorzugehen, dass zwischen der Intensität des Hemmungsreizes und der Grösse des dadurch eben zu hemmenden Unterschiedes ein proportionales Verhältniss besteht; so zwar, dass der Factor, mit welchem die Intensität des Hemmungsreizes multiplicirt werden muss um die Grösse des dadurch eben zu hemmenden Unterschiedes zu finden (ich sage kurz: der Hemmungscoefficient) unter den angedeuteten Versuchsumständen 0.035 beträgt. Dieses Ergebniss legt nun die Vermuthung nahe, dass die Thatsachen des Weber'schen Gesetzes, sofern dasselbe sich auf ebenmerkliche Unterschiede bezieht, einfach einen Specialfull des Hemmungsgesetzes darstellen. Bei den Versuchen zur Prüfung des Weber'schen Gesetzes werden ja die Unterschiede zweier simultanen oder successiven Reize, während der Einwirkung beider oder mindestens Eines derselben, bestimmt; und es lässt sich denken, dass diese Vergleichsreize selbst, der Auffassung ihres Unterschiedes gegenüber, als Hemmungsreize auftreten sollten. Dadurch würde sich aber das proportionale Verhältniss zwischen Reizintensität und Unterschiedsschwelle, welches das Weber'sche Gesetz statuirt, in einfachster Weise, und ohne Zuhülfenahme der Fechner'schen Hypothese eines logarithmischen Verhältnisses zwischen Reiz und Empfindung, erklären lassen. Um eine Vergleichung zwischen den Thatsachen der Hemmung |