194 



Scientific Proceedings, Royal Diihlin Society. 



ludian ink mixed with water, and allowed to stand for forty-eight hours ; 

 the coarser particles are then deposited, and the supernatant liquid is 

 employed. 



The apparatus is shown in fig. 2 and in section in fig. 3. A variable depth 

 of the liquid is obtained by the movement of a plunger with glass bottom H, 

 fig. 3, whicli can be gradually immersed within a cylinder or cistern I, also with 

 a glass bottom, containing the absorbing liquid. Light is reflected upwards 



through the cylinder by means of a mirror, M, at 45°. The amount of light 

 scattered from various large surfaces can thus be very easily compared by the 

 relative depth of liquid required to produce extinction. The pillar L Is 

 graduated, and the cistern I raised or lowered by tiie rack and pinion U. In 



