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 |