K. 346, 347.] I. Normal and Anomalous Colour Systems 411 Perhaps connected with the difficulty of recognizing the right colour, is another peculiarity that Nagel found to be very notice able in all anomalous persons. This was what appeared to be an accentuation of certain contrast phenomena. For example, a yellow field viewed by itself is described correctly as yellow; but if it is displayed alongside a red field, it is then called green. This is the explanation of what is practically a very important fact, namely, that in observing two adjacent objects of different colours, anomalous persons are liable to make big mistakes that would be entirely out of the question in normal vision. Thus a gaslight or electric light of the usual pale yellow colour is taken for green if it is put by the side of a red light. Note by v. Kries (Prepared specially for insertion here, January 1924) In recent years extensive studies have been made on the connections between the various forms of anomalous colour vision. Hess1, in particular, has carried out very thorough investigations on this subject. The writer will limit himself here to a brief discussion of the matter so far as it has to do with typical dichromats (protanopes and deuter- anopes). The old view, as represented by Seebeck and Helmholtz, namely, that the cases of congenital partial colour blindness were arranged in two typically different groups not connected by any transitions, had been, as stated above, fully established by systematic testing of colour-mixture equations. Hess has again challenged this fact. The particular support on which he relies is that even within each group there are considerable differences in the mode of vision. However, Hess’s observations were not made under conditions that insure pure daylight vision, and for this reason alone decisive importance cannot be attached to them. For the point that is of theoretical interest is, whether or not there is a distinct difference between the mechanisms of daylight vision in the protanopic and deuteranopic visual organs. This question can only be decided by observations where precautions have been taken to insure practically pure cone vision, by having the eye light-adapted and the observation confined to a small field. If, for 1 y. Hess, Die Rot-Grün-Blindheiten. Pflügers Archiv, CLXXXV. 1920.—Idem, Die angeborenen Farbensinnstörungen und das Farbengesichtsfeld. Archiv, f. Augenheilkunde, LXXXVI. 1920. — v. Hess gave a very thorough account of the subject in his article on “Farbenlehre” in the Ergebnissen der Physiologie, Bd. XX. p. 1. 1922. See y. Kries’s re¬ view of this article, Zur physiologischen Farbenlehre, Klinische Monatsblätter f. Augen¬ heilkunde, LXX. p. 577. 1923.