104 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



obtained by the movement of a plunger with glass bottom H (fig. 15) 

 which 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 



jS 



^ 



Fig. 15. 



easily compared by the relative depths of liquid required to produce 

 extinction. The pillar L is graduated, and the cistern I raised or 

 lowered by the rack-and-pinion K. In order to exclude extraneous 

 light the observer rests his forehead in a shaped head-rest A, and a 

 black cloth covers the head. After one minute the eye attains a fairly 

 steady state, and either eye can be used at pleasure by sliding the head- 



