DIRECT 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.
Chart 1.
Chart 2.
Chart 3.
Chart 4.
Chart 5.
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.
Charts 3, 4 and 5 deal with experiments, with one, two and three standard candles of the London Gas Referees. The curves represent the physiological rate of declining luminosities by distance. Some characteristic differences from other lights are brought together in Charts 4 and 5. The slight irregularities in the curves are probably due to the readings being made at half unit intervals. These acting sometimes in opposite directions, fully account for want of symmetry in the curves.
A noticeable feature in these experiments is the small amount of physiological luminosity added by each successive candle to the first. Theoretically, if one candle equals 21 units intensity, two should equal 42, and three 63, whereas the physiological additions of luminosity are not only much less, but vary with different luminous intensities, as will be seen by the following comparisons:—
| 1 Candle | 2 Candles | 3 Candles | Total. | ||||
| Theoretical | 21 | + | 21 | + | 21 | = | 63 |
| Physiological | 21 | + | 3 | + | 6 | = | 30 |
Chart 4 represents the Luminosity of gas from a batswing burner reduced to one, two and three candle power at one foot distance.
| 1 Candle | 2 Candles | 3 Candles | Total. | ||||
| Theoretical | 21 | + | 21 | + | 21 | = | 63 |
| Physiological | 21 | + | 3 | + | 7 | = | 31 |
Chart 5 represents the Luminosity of a hand Paraffin Lamp similarly reduced.
| 1 Candle | 2 Candles | 3 Candles | Total. | ||||
| Theoretical | 21 | + | 21 | + | 21 | = | 63 |
| Physiological | 21 | + | 3 | + | 6 | = | 30 |