— 234 — 



For estimations of the relative intensity of light the apparatus 

 can be used without further measures. Althought the standard co- 

 lour is not quite stable, it is of no significance in that respect. When 

 for instance the relative illumination in a forest is to be estimated, 

 (viz. the illumination in the forest in percent of the free daylight) 

 an estimation is at first carried out in an open place and later in 

 the forest. Since the light in the forest is not uniform, the exposure- 

 meter should be carried around in the forest during the estimation 

 (Boysen Jensen 1910 p. 12). The alteration of the standard colour 

 during 2 consecutive estimations is imperceptible. 



On the other hand the apparatus can not be used directly for 

 absolute estimations of the intensity of light. That is only possible 

 by preparing an unalterable standard colour that can easily be 

 reproduced. This I have so far not succeeded in doing. When 

 therefore absolute estimations are to be carried out with the 

 apparatus described above, it is necessary to compare the 

 apparatus with that of Wiesner and to express the estimations 

 in Bunsen's units. Such a comparison should be carried out from 

 time to time and always under the same conditions. I do so in 

 direct sunshine. An intensity of light that can darken Bunsen's 

 silver chloride paper to the standard colour in 1 sec. is taken as 

 being 100. 



I have determined the intensity of light in the different hours 

 of 5 days in the months of July and August, partly with and partly 

 without sunshine. The mean of this estimations that give only a 

 rough idea of the daily course of the intensity of light in these 

 months, is as follows: 



3. Estimation of the respiration. The amount of G0 2 given 

 off by the plants in respiration can be measured by the apparatus 

 mentioned above. More easily the determinations can be made with 

 the following apparatus. The plant is enclosed in a glass jar, closed 

 with a glass lid or with a cork stop impregnated with paraffin. 

 The dimensions of the jars should be in proportion to the intensity 



Hours, 

 12—1 



1— 2 



2— 3 



3— 4 



4— 5 



5— 6 



6— 7 



7— 8 



intensity of light. 



39 

 38 

 36 

 33 

 29 

 22 

 14 

 4 



