Some experiments in the measurement of the colour fields in the horizontal direction with the pure spectrum colours will help to show what importance is to be attached to the luminosity of the colour and the size of the spot of light with which the observations are made. A yellow and a blue of the spectrum were taken of such hues that when mixed they formed a patch of white light similar to the electric light. Their luminosities were measured, and the yellow found to be 1·6 of the light of an amyl-acetate lamp or 1·28 standard candles; the blue was 1/24 of this luminosity. The fields for these two colours were measured by automatically throwing spots of each colour separately on a white card which moved round a centre over which the eye was placed. The light was subsequently diminished to ½, ¼, and ⅛ of the above values, and readings again made. The following results were obtained with a spot of ·7 inch diameter:-- Yellow. Blue. Light. -------------------------- --------------------------- Nasal side. Temporal side. Nasal side. Temporal side. Full 33° 45° 35° 45° ½ 24° 36° 26° 38° ¼ 18° 24° 22° 32° ⅛ 11° 15° 19° 30° With a spot of ·3 inch diameter the following were obtained:-- Full 24° 32° 21° 27° ½ 17° 28° 16° 22° ¼ 13° 16° 14° 20° ⅛ 8° 10° 13° 16° It will be evident how the field contracts as the light is diminished in brightness, and also that the blue field does not diminish equally with the yellow field, but is more persistent. Again, it will be noticed that the luminosity of the blue, for the same extent of field to be covered, has to be much lower than for the yellow. The diminished area of the spot of light also diminishes the field, and the same order of diminution of field is obtained as with the larger spot. Another set of experiments, made with the same aperture of slit passed through the spectrum, and the field taken at different points, give the following results:-- Spectrum scale. Nasal side. Temporal side. (See Fig. 41, page 210.) 58·6 18° 35° 54·6 27° 46° 50·6 33° 47° 46·6 25° 30° 42·6 21° 21° 38·6 17° 17° 34·6 22° 30° 30·6 25° 33° 26·6 33° 40° 22·6 37° 44° 18·6 28° 40° 14·6 22° 34° 8·6 20° 30°