The distances, roughly measured, of the neutral point from the lamp, corresponding to the foregoing series of observations, were these: 1st observation 2 feet 2 inches. 2nd observation 2 feet 6 inches. 3rd observation 2 feet 10 inches. 4th observation 3 feet 2 inches. 5th observation 3 feet 7 inches. 6th observation 4 feet 6 inches. At the end of this series of experiments the direction of maximum polarisation had again become normal to the beam. The laboratory was next filled with the fumes of gunpowder. In five successive experiments, corresponding to five different densities of the gunpowder-smoke, the angles enclosed between the line of vision to the neutral point and the axis of the beam, were 63 degrees, 50°, 47°, 42°, and 38° respectively. After the clouds of gunpowder had cleared away, the laboratory was filled with the fumes of common resin, rendered so dense as to be very irritating to the lungs. The direction of maximum polarisation enclosed, in this case, an angle of 12°, or thereabouts, with the axis of the beam. Looked at, as in the former instances, from a position near the electric lamp, no neutral point was observed throughout the entire extent of the beam. When this beam was looked at normally through the selenite and Nicol, the ring-system, though not brilliant, was distinct. Keeping the eye upon the plate of selenite, and the line of vision perpendicular, the windows were opened, the blinds remaining undrawn. The resinous fumes slowly diminished, and as they did so the ring-system became paler. It finally disappeared. Continuing to look in the same direction, the rings revived, but now the colours were complementary to the former ones. _The neutral point had passed me in its motion down the beam, consequent upon the attenuation of the fumes of resin_. With the fumes of chloride of ammonium substantially the same results were obtained. Sufficient, however, has been here stated to illustrate the variability of the position of the neutral point. [Footnote: Brewster has proved the variability of the position of the neutral point for skylight with the sun's altitude, a result obviously connected with the foregoing experiments.]