﻿386 Royal Society : — Prof. Tyndall on the Blue Colour 



diculai to the illuminating beam, the quantity of light that reached 

 the eye was in all its positions a maximum ; when the crystallogra- 

 phic axis was parallel 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 

 discharged, the direction of maximum polarization being at right an- 

 gles 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 co- 

 lours 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 thick- 

 ness from the centre outwards. When placed in its proper position 

 between the Nicol and the cloud, it exhibited 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 substances 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 ieeble result of this action, may, by placing it in 

 proximity with another gas or vapour, be caused to exhibit under 

 light vigorous, if not violent action. The case is similar to that of 

 carbonic acid gas, which diffused in the atmosphere resists the de- 

 composing action of solar light, but when placed in contiguity with 

 the chlorophyl in the leaves of plants has its molecules shaken 

 asunder. 



Dry air was permitted to bubble through the liquid nitrite of butyl 

 until the experimental tube, which had been previously exhausted, 

 was filled with the mixed air and vapour. The visible action of light 

 upon the mixture after fifteen minutes' exposure was slight. The 

 tube was afterwards filled with half an atmosphere of the mixed air 

 and vapour, and another half atmosphere of air which had been 

 permitted to bubble through fresh commercial hydrochloric acid. 

 On sending the beam through this mixture, the action paused barely 

 sufficiently long to show that at the moment of commencement the 

 tube was optically empty. But the pause amounted only to a small 

 fraction of a second, a dense cloud being immediately precipitated 

 upon the beam which traversed the mixture. 



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

 polarization. This is still an undecided point ; hut 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. 



