— 233 — 



the same as that of the ordinary paper, but it blackens slowly and 

 very slightly. By exposing it for 1 — 2 hours in direct sunshine it gets 

 a colour that alters only slightly on further exposure. This colour 

 can therefore, under conditions that I shall mention below, be used 

 as a standard colour. After an exposure of a few seconds in direct 

 sunshine the ordinary Rhodamin B paper will acquire precisely 

 the same tone. 



Further it is necessary to prepare an apparatus that can be 

 used for the exposure 

 of the sensitive paper. 



1 use an apparatus of 

 similar construction 

 to that of Wiesner 

 (1907). It consists of 

 3 sheets of card-board 

 of the same size, stuck 

 together along one of 

 the edges. In the 2 

 upper sheets a rect- 

 angular aperture (1 X 



2 cm) is cut out. A 

 strip of tbe paper with 

 the standard colour is 

 arranged between the 

 two upper sheets, co- 

 vering half of the 

 aperture, while the 

 sensitive paper is put 

 in between the two 



lower sheets, covering the other half part of tbe aperture. By pull- 

 ing the end of the sensitised paper new parts of it can be brought 

 before the aperture. When the whole strip is exposed, it can be re- 

 placed with fresh strips without opening the apparatus. A band of 

 india rubber holds the sheets together. 



As mentioned above the Rhodamin B paper is sensitive to the 

 red part of the spectrum. Still the sensibility to the blue rays is 

 great. It is therefore an advantage to remove a part of the blue 

 rays by filtering the light to be estimated through a yellow screen 

 in the same way as is done in photography. As yellow filter I use 

 a 0.025 % solution of potassiumbichromate in a thickness of 1 cm. 

 It is more convenient and for many purposes sufficiently accurate, to 

 use films coloured with picric acid. Yet such a filter is not very stable. 



Botanisk Tidsskrift. 36. Bind. 16 



Yellow- 

 screen 



'/////////////////////////. 



m 



Standard - 

 paper 



uuuu 



India rubber 

 band 



- Rhodamin ß 

 paper 



Fig. 2. Exposuremeter. 



