It has no photographic action on the six sensitised papers dealt with in the photographic section. LIGHT INTENSITIES. The apparatus for determining the unit values of light intensities in the following series of measurements, consisted of a conical rectangular hopper tapering from 2 feet to 2 inches square. This was adapted so that the light from the small end, commanded the stage of the optical instrument sufficiently close to cut off outside light. The wide end facing a north sky was adapted with sliding shutters, to regulate the area of incident light; of the six water-colour pigments which nearest corresponded to the standard colours, washed to their full depth on Whatman’s paper, six measurements were made. These measurements are shown in Table IV, and classified in Table V. It will be noted that the readings are constant for all the colours between 16 and 26 units, except a variation of light ·15 in the 24-inch opening, which is in effect as if the cone was not present, and ·2 in the 8-inch area of orange. Note.--Experiments in this branch give some information relating to the perception of colour under daylight conditions, by limiting the range of intensities within which colour can be distinguished and differentiated, whilst their separate photographic action (page 48) suggests the impression that colour phenomena, outside these limits, may be a physiological expression of widely varying underlying energies. TABLE IV. -------------+---------+----------+--------+--------+---------+------- | Square | | | | | Pigment. | Inches | Light | Black. | Red. | Orange. | |Aperture.|Intensity.| | | | -------------+---------+----------+--------+--------+---------+------- Carmine | 2 | 10 | ·5 | 20·2 | ·3 | ---- " | 4 | 11 | ·5 | 19·1 | ·4 | ---- " | 6 | 14 | ·46 | 18·95| ·59 | ---- " | 8 | 16 | ---- | 16·9 | 1·1 | ---- " | 10 | 20 | ---- | 16·9 | 1·1 | ---- " | 12 | 22 | ---- | 16·9 | 1·1 | ---- " | Open | 26 | ---- | 16·9 | 1·1 | ---- -------------+---------+----------+--------+--------+---------+------- | | | | Yellow | | Lemon Yellow | 2 | 10 | ---- | 6·9 | ·1 | ---- " " | 4 | 11 | ---- | 6·9 | ·1 | ---- " " | 6 | 14 | ---- | 6·9 | ·1 | ---- " " | 8 | 16 | ---- | 7·0 | ---- | ---- " " | 10 | 20 | ---- | 7·0 | ---- | ---- " " | 12 | 22 | ---- | 7·0 | ---- | ---- " " | Open | 26 | ---- | 7·0 | ---- | ----