ARTIFICIAL SKY 119 



gas by light. A faint bluish cloud, due it may be, or it 

 may not be, to the residue of some vapor previously em- 

 ployed, was formed in the experimental tube. On look- 

 ing 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 vertical 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 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 angles to the illuminating beam; the plane of 

 vibration of the polarized light was perpendicular to the 

 beam. ' 



Thin plates of selenite or of quartz, placed between 

 the Nicol and the actinic cloud, displayed the colors of 

 polarized light, these colors being most vivid when the 



1 This is still an undecided point ; but the probabilities are so much in its 

 favor, 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. 



