ABSTRACT
Even though colours of quite different modes of appearance may, through reduction, be transformed into film colours, that does not mean that a film colour is somehow “ contained ” in a colour-impression of some other mode of appearance. A surface colour is not a modification of a film colour, nor does the latter form a foundation upon which a volume colour might be built up, etc. It is true, of course, that lustre appears only in conjunction with colours of other modes of appearance, and that transparent colours presuppose surface colours to be seen through them. To this extent it ma y be said that they do not occur independently of others, but they are not strictly modifications of these other colour-impressions. We have genuine modifications of colour only in the case of surface colours appearing under the influence of changes in illumination. All the following experiments are devoted particularly to these. Let us begin with achromatic surface colours in achromatic illumination. The colour-series which has hitherto been studied with special interest is the one called by Hering the series of achromatic colours. 1 “ We may think of all the achromatic colours as so arranged in series that we find at the one end-point the purest conceivable black, at the other the purest white, and between them in a continuous sequence all possible degrees of darkness or brightness of black, grey-black, black-grey, grey, white-grey, grey-white and white.” 1 v. Kries agrees with Hering’s view of the unidimensionality of the continuum of achromatic colours. “ There is no modification of grey in which the black and white remain in the same ratio and in which both are uniformly stronger or weaker.” 2 G. E. Müller has expressed the same view in his discussions of the quantitative unidimensionality of the black-white series of sensations. Indeed, all the authors who have dealt with the black-white series agree that this series has only one dimension. This view is justified if we limit ourselves to the film or surface colours which may be produced in one particular illumination ; for all film or surface colours appearing in one fixed illumination may actually be arranged in one series. If, however, we permit the achromatic illumination to vary freely in intensity, we discover a bidimensionality of achromatic surface colours.
