CHARACTERISTIC DIFFERENCES IN RESPONSE 171 



the reflected spot of light is correspondingly quickened. 

 The less effective rays, on the other hand, cause a relatively 

 slow movement. The records of the different rates of move- 

 ment of the spot of light thus give a vivid picture of the 

 relative effectiveness of the different rays. A continuous 

 record of the response may be obtained photographically 



Fig. 47. Records of Response under Sunlight (S), Red Light ( R), 

 Blue Light (B), and Sunlight once more (S) (up-stroke 

 representsflotation) 



Successive dots at intervals of 30 seconds. Note the slow rate 



under blue light (B) . 



by means of sensitised paper on the recording drum ; it 

 can, however, be obtained more simply in the following 

 manner. A split brass tube clasping a vertical rod r (fig. 46) 

 acts as a spring and carries a writing-pencil w. The 

 excursion of the spot of light up or down is followed by the 

 hand, a thick dot being made at intervals of 30 seconds. 



Fig. 47 gives records of responses under direct sunlight 

 (S), under red light (R), and under blue light (B). A final 

 record under sunlight was taken to make sure that the 



