along another line, say 300° to 120°, and other points noted and marked on the chart. When the whole circle has been examined, the various points are joined together, and we have the boundary of vision for a white object. The boundaries of the _colour_ perception for (say) small red and green discs are found in the same way. The former is depicted in the left-hand chart and gives the field for the right eye, and the latter with that for white in the right-hand chart for the same eye. It will be noticed that two boundaries are given, one taken at mid-day and the other at 6 p.m. The brighter the colour, the larger is the boundary in both cases, showing that the field of colour vision varies according to the illumination. Now it is difficult from this method of experimenting to determine whether the fields for different colours are the same or differ in extent, as we have no information as to whether the colours themselves which were used were physiologically equal. The only way by which this can be satisfactorily determined is by using spectrum colours each of known brightness and area. (Some preliminary experiments made by myself regarding the colour fields will be found in the appendix, and will be referred to later.) It must not be thought that the various colour boundaries mark the limit at which _light_ is perceived, but only the limit at which colour is seen; outside the boundaries the objects appear of a nondescript colour, to which we shall by-and-by call attention. The yellow spot lies within the circle of 5°, and the blind spot on which no sensation of light is stimulated is shown by the black dot about 15° away from the centre. I have only attempted to sketch, in unphysiological language, the primary apparatus with which our experiments in colour have perforce to be made. CHAPTER II. It will be seen, then, that in measuring colour or light several circumstances have to be taken into account. These are not simple, and require differentiating one from another before the results of colour measures can be finally laid down as correct, or as being held to be applicable to all cases.