224 



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



With this explanation, the term " cloud," or " incipient cloud," as I pro- 

 pose to employ it, cannot, I think, be misunderstood. 



I had been endeavouring to decompose carbonic acid gas by light. A 

 faint bluish cloud, due it may be, or it may not be, to the residue of some 

 vapour previously employed, was formed in the experimental tube. On 

 looking across this cloud through a Nicol's prism, the line of vision being 

 horizontal, it was found that when the short diagonal of the prism was ver- 

 tical, the quantity of light reaching the eye was greater than when the long 

 diagonal was vertical. 



When a plate of tourmaline was held between the eye and the bluish 

 cloud, the quantity of light reaching the eye when the axis of the prism 

 was perpendicular to the axis of the illuminating beam, was greater than 

 when the axes of the crystal and of the beam were parallel to each other. 



This was the result all round the experimental tube. Causing the crystal 

 of tourmaline to revolve round the tube, with its axis perpendicular 

 to the illuminating beam, the quantity of light that reached the eye was 

 in all its positions a maximum. When the crystallographic axis was par- 

 allel to the axis of the beam, the quantity of light transmitted by the crystal 

 was a minimum. 



From the illuminated bluish cloud, therefore, polarized light was dis- 

 charged, the direction of maximum polarization being at right angles to the 

 illuminating beam ; the plane of vibration of the polarized light, moreover^ 

 was that to which the beam was perpendicular*. 



Thin plates of selenite or of quartz, placed between the Nicol and the 

 bluish cloud, displayed the colours of polarized light, these colours being 

 most vivid when the line of vision was at right angles to the experimental 

 tube. The plate of selenite usually employed was a circle, thinnest at the 

 centre, and augmenting uniformly in thickness from the centre outwards. 

 When placed in its proper position between the Nicol and the cloud, it ex- 

 hibited a system of splendidly coloured rings. 



The cloud here referred to was the first operated upon in the manner 

 described. It may, however, be greatly improved upon by the choice of 

 proper substances, and by the application in proper quantities of the sub- 

 stances chosen. Benzol, bisulphide of carbon, nitrite of amy], nitrite of 

 butyl, iodide of allyl, iodide of isopropyl, and many other substances may be 

 employed. I will take the nitrite of butyl as illustrative of the means 

 adopted to secure the best result with reference to the present question. 



And here it may be mentioned that a vapour, which when alone, or 

 mixed with air in the experimental tube, resists the action of light, or shows 

 but a feeble result of this action, may, by placing it in proximity with an- 



* I assume here that the plane of vibration is perpendicular to the plane of polariza- 

 tion. This is still an undecided point ; but the probabilities are so much in its favour, 

 and it is in my opinion so much preferable to have a physical image on which the mind 

 can rest, that I do not hesitate to employ the phraseology in the text. Even should 

 the assumption prove to be incorrect, no harm will be done by the provisional use of it. 



