100 Forestry Quarterly 



Thinning increases the increment of individual trees, but of 

 course reduces the number of trees. Accordingly for best results 

 it cannot be carried too far. There is then for each stand an 

 optimum. For beech, Michaelishas found this such as maintains 

 a crown density of 0.7 to 0.8. The best practice in thinning is 

 such as results in a stand with the most valuable increment possible 

 under the given environment. The utilization of these site factors, 

 one of which is light, will finally lead to the only definite solution 

 of the thinning problem. 



Various investigators have measured various physical quantities 

 in their study of light in the forest. Some have used the polari- 

 zation photometer, some selenium cells ; some photosensitive paper. 

 None have discussed the question whether their instnmients were 

 sensitive in the same way as the green leaf. The problem simmers 

 down to a study of the composition of light before and after 

 passing through the crown of a stand. 



Absorption and assimilation of Ught are not necessarily the 

 same thing while reflection enters to complicate matters. 



Most studies of Ught have been made on the ground imder stands 

 and the light measm'ed compared with the average foimd in open 

 fields. Reflected Hght is here lost track of. The true state of 

 affairs can only be learned by simulanteous measurements of light 

 above the forest and at different levels within and beneath the 

 crowns. Moreover, it is only reasonable to conclude that some 

 light penetrates the crown unaltered. 



The author's comparative studies have been made in stands 

 of beech and spruce of the same age and growing at the same 

 altitude, on the same soil and expostire and so close together as to 

 receive the same precipitation. The thinning alone varies and 

 this is controlled arbitrarily. 



Such an introductory recognition of the inherent difficulties of 

 the problem would seem to warrant high expectation of better 

 results. Such expectations fall when the author selects the so- 

 called radiometer of Crooke's to measure light. The choice 

 appears to have been made because of convenience of manipulation, 

 and there is no discussion of its advantages over other instru- 

 ments hererofore used. 



Each radiometer is calibrated against a standard candle. 

 Plotting speed of revolution against intensity of light the results 



