l88 CHAP. XXII. EFFICIENCY OF THE SPECTRAL RAYS 



The Heterostatic Method. — The principle of this method 

 of enhancing the sensitiveness of physical instruments is of 

 much interest. A feeble electric charge cannot be easily 

 detected ; but by the previous charging of the indicating 

 needle its deflection under a feeble electric charge in a 

 neighbouring conductor becomes greatly increased. For 

 physiological investigations we may secure greater sensi- 

 tiveness by a somewhat analogous process, though the 

 antecedent physical and physiological treatments are very 

 different. 



The Threshold of Response. — Let us consider the par- 

 ticular condition of the photosynthetic organ when it fails 

 to respond to feeble incident radiation. The cause of this 

 inefficiency is that a certain amount of energy must be 

 absorbed before the plant-organ can manifest an external 

 response by the evolution of oxygen. There is thus a pre- 

 paratory limit, which I shall, for want of a better term, call 

 the Verge or Threshold of Response. The energy acting on 

 the plant below this is outwardly inefficient, but it has brought 

 the plant to the verge of response ; the preliminary work has 

 been accomplished, so that radiation, previously ineffective, 

 will now precipitate response increasingly as it increases in 

 intensity. 



The Auxiliary Stimulator. — The plant is brought to the 

 verge of response by the employment of a 30-c.p. Pointolite 

 for the auxiliary stimulation. The incandescent lamp is 

 enclosed in a conical tube mounted on a suitable stand, so 

 that by a slight inclination of the tube the rays may be 

 thrown upon the plant-vessel (fig. 50) ; the lamp-tube is so 

 fixed that it does not obstruct the spectral rays falling on 

 the plant. The intensity of radiation is varied by moving 

 the auxiliary light towards or away from the plant. It is 

 so adjusted at first that bubbling is initiated. The source 

 of light is now removed slightly away from the plant till 

 bubbling just ceases. This is the adjustment for the limiting 

 verge. There is no photosynthesis, but the plant is ready 

 to exhibit it in response to additional radiation however 



