ARTIFICIAL SKY. Ill 



ployed are sufficiently attenuated, no matter what the 

 liquid may be, the visible action commences with the 

 formation of a blue cloud. But here I must guard 

 myself against all misconception as to the use of this 

 term. The ' cloud ' here referred to is totally invisible 

 in ordinary daylight. To be seen, it requires to be 

 surrounded by darkness, it only being illuminated by a 

 powerful beam of light. This blue cloud differs in 

 many important particulars from the finest ordinary 

 clouds, and might justly have assigned to it an inter- 

 mediate position between such clouds and true vapour. 

 With this explanation, the term ' cloud,' or ' incipient 

 cloud,' or 'actinic cloud,' as I propose 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 pre- 

 viously 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 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. 



