170 CHAP. XX. EFFICIENCY OF DIFFERENT COLOURS 



found to transmit light which was practically monochro- 

 matic. The red glass transmitted rays the wave-lengths of 

 which extended from 760 to 605 W ; the wave-lengths of 

 light transmitted through the blue glass extended from 

 475 to 400 W, i.e. to about the G line. The method of 

 experimental procedure was to determine the period of 

 flotation under (1) direct sunlight, (2) under transmitted 

 red light, and (3) under transmitted blue light. 

 The following is a summary of the results : 



Period of successive flotations under sunlight, 50 seconds. 



„ red light, 175 

 „ blue light, 400 



The relative activities are inversely proportional to the 

 respective periods of flotation. Hence — 



S : R : B = — : — : — 



50 175 400 



In other words, taking the activity under blue light as 

 1, that under red light is 2-3 ; under direct sunlight it is 8. 



B:R:S: : 1 :2-3:8 . . . (1) 



The Characteristic Differences in Response under 



Different Colours 



The periods of flotation thus measure the relative activity 

 under different lights ; but they give us only the final 

 results, not the entire history of the photosynthetic action. 

 More complete and detailed information can be obtained 

 by means of a continuous record of the rate of flotation. 

 This is secured by an application of the optical method, 

 already referred to. The spot of light reflected from the 

 mirror attached at the fulcrum of the Torsion Balance 

 moves downwards (represented up in the record) during 

 the increasing buoyancy due to the formation of the bubble 

 of oxygen. When this takes place rapidly under exposure 

 to the more effective light, the rate of down-movement of 



