One object of these experiments was to obtain more insight into the physiological conditions of light, as bearing on the question of standardization.
The first two experiments are records of intensity changes by time, in Morning and Evening light, and are of interest, as bearing on the lowest luminosities for reading, for viewing objects at different distances, and for defining the limits at which colours visually disappear. The measurements are marked in neutral tint units and plotted in curves in Charts 1 and 2.
The numbers on the single curve in the Morning chart represent total absorption of the direct light 20 degrees above the horizon. The upper curve in the Evening chart also represents the direct light, whilst the under curve represents the light values as reflected from a lime sulphate surface; except in the case of the reading notes when it represents the printed paper surface. The difference between the two curves is the loss of light incident on reflection, but this must not be rigidly interpreted for all cases, as there is reason for supposing it varies with different lights.
In measuring the physiological intensity of direct lights, the presence (activity) of the unabsorbable red ray, prevents their being dealt with directly by the absorptive method. Such lights can, however, be made measurable by a sufficient diffusion, as already explained in the case of direct sunlight, the proportion of diffusion required, being more or less according to the intensity of the light; in the following examples, one reflection from a white surface or from the ordinary grease spot arrangement, was found to be sufficient.
Note.--Light reflected from grease is not above suspicion, it being governed by the law of specific absorption already dealt with.
_Upper Curve 20° above Horizon Lower Curve as Reflected from a 1 Foot Square Lime Sulphate Screen_ ]
Luminous Intensities of _1, 2 & 3 Standard Candles_.]
_Reduced to Equal 1, 2 & 3 Candles at 1 Foot._]
_Reduced to 1, 2 & 3 Candles at 1 Foot._]
The experiments were carried out on a home-made twelve-foot photometer, with the usual protected lantern at each end, one being removed for present purposes. The grease spot carrier was replaced by a one-foot square diaphragm, with a standard white surface; this travelled the whole length of the photometer at right angles to the light, and the readings made at one-foot intervals at an angle of 45 degrees, 10 inches distance from the white surface.
