158 
The lecturer here paused to exhibit Newton’s experiment of 
the decomposition of a beam of white light into its constituent 
coloured rays, and referring to a diagram shewed that the energy 
of the waves which produced the sensation of red was many 
times as great as that of those which produced blue. Then 
naturally we should expect that the blue vibrations (using this 
term for the sake of brevity), having least energy, would be 
soonest stopped by particles floating in the medium through 
which the light passed. That this was so in fact the next ex- 
periment clearly shewed: the cloud produced in the vapour of 
nitrite of butyl (though not so clearly produced as on some 
previous occasions), was distinctly visible to those who were 
near the table, and was of a blue or violet shade. The light 
emitted from the cloud was moreover perfectly polarized, as was 
tested by the interposition between it and the eye of a crystal 
of tourmaline. The next experiment, owing to some defect in 
the apparatus, the preparation of which had been necessarily 
hurried, did not succeed, but was described by the Professor as 
the excitation of a real cloud within the tube containmg an 
actinic cloud and the reproduction on a small scale of the azure 
of the sky. It was, as he described it, taking a piece of the sky 
and producing in limited space all the phenomena of cloud light 
and volarization which are produced by the sun light and the 
clouds of heaven. 
To shew the exceeding minuteness of the particles to which 
the blue colour of the sky is due, Professor Tyndall described 
the difficulty of getting rid of the residue of vapour in the ex- 
perimenting tube, a difficulty which had more than once led him 
to false conclusions until the experiment which the residuary 
vapour vitiated had been repeated. The last experiment was an 
examination of the vapour by means of polarized light, in which 
the cloud, distinctly visible in one direction, was entirely lost to 
sight when the polarizing prism had been revolved through an 
angle of forty degrees. 
