230 



Prof. Tyndall on the Blue Colour of the Sky, [Jan. 14j 



The windows of the laboratory were now opened for some minutes, a 

 portion of the incense smoke being permitted to escape. On again 

 darkening the room and turning on the beam, the line of vision to the 

 neutral point was found to enclose with the axis of the beam an angle of 

 63°. 



The windows were again opened for a few minutes, more of the smoke 

 being permitted to escape. Measured as oefore the angle referred to was 

 found to be 54°. 



This process was repeated three additional times ; the neutral point was 

 found to recede lower and lower down the beam, the angle between a line 

 drawn from the eye to the neutral point and the axis of the beam falling 

 successively from 54° to 49°, 43° and 33°. 



The distances, roughly measured, of the neutral point from the lamp, 

 corresponding to the foregoing series of observations, were these : — 



1 st observation 2 feet 2 inches. 

 2nd „ 2 „ 6 „ 

 3rd „ 2 „ 10 „ 

 4th „ 3 „ 2 „ 

 5th ,, 3 ,, 7 

 6th „ 4 „ 6 „ 



At the end of this series of experiments the direction of maximum pola- 

 rization 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 gun- 

 powder smoke, the angles enclosed between the line of vision to the neutral 

 point and the axis of the beam were 63°, 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 my lungs. The direction of maximum polarization enclosed in this case 

 an angle of 1 2°, 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 normal, 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 along the perpendicular, 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 f umes of resin. 



In the fumes of chloride of ammonium substantially the same results 

 were obtained as those just described. Sufficient I think has been here 

 stated to illustrate the variability of the position of the neutral point. The 



