52 LIGHT, VEGETATION AND CHLOROPHYLL 



or less diffused fashion. In the first case, the fight is fike direct 

 sunfight; in the second, it is fike the diffuse fight from a blue 

 or cloudy sky, and we may remember in this connection that 

 the sky is more often covered than clear. 



In each case the measured reflection factor is different. 

 Fortunately, the differences are not great, either for reflection 

 or for transmission; according to the measurements made by 

 Seybold, they have a maximum value of 2 per cent in the 

 transmission factor. 



Finally, everyone knows that different species of vegetation 

 have differently coloured leaves and that a very young leaf 

 differs from an adult leaf and from a withered autumn leaf. 

 It is therefore necessary to work, not only with the correct 

 methods but also on well-specified samples, so that even this 

 question — undoubtedly the simplest that will be treated in 

 this book — appears complex enough when it is examined in 

 some detail. 



Results 



To show how little confidence can be placed in measure- 

 ments made under conditions which are not absolutely 

 correct, we may quote the results obtained by two experi- 

 menters using yellow fight with the leaves of Fraxinus excelsior ; 

 one gives a transmission factor of 5 per cent, the other of 

 10 per cent; for Tilia parviflora, one gives 1 per cent, the 

 other 17 per cent. This is an extreme case but is taken from 

 the measurements generally considered as being the best. 



It seems probable that the most serious errors were not 

 due to the nature of the light, which was sufficiently mono- 

 chromatic, but rather to a defective method of measuring 

 the diffused fight; even today, and in many fields of work, 

 too many experimenters underestimate the dangers of an 

 imperfect method in the study of diffusing surfaces. 



In the visible region, as we might expect simply because 

 the leaves are green by reflection and by transparency, the 

 transmission and reflection factors are both maximum in the 

 middle of the spectrum, in the green, at the wave-length of 



