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be multiplied with 100 ; so in this case the intensity of illumination 

 is 20. The time, that is used for colouring the paper, is measured 

 by a stop-watch. 



When a photographic paper is to be used for such estimations, 

 it is necessary that I .t = constant (7 = the intensity of illumination, 

 t the time to colour the paper). For the paper used in Wynnes 

 Photometer, this seems to be the case with illuminations of greater 

 intensity; with feeble intensities the time that is used for colouring 

 the paper seems to be somewhat greater than the theoretical time. 



The paper used in Wynnes Photometer is like most photographic 

 papers more sensitive to blue than to red light. This error I have 

 tried to diminish by covering the Photometer with a yellow glass 

 to absorb part of the blue rays. Still Wynnes' photometer is 

 not satisfactory, and in my last experiments I have used another 

 paper, Rhodamin B paper, to estimate the intensity of illumination. 

 The advantage of this paper is that it is very sensitive to the red 

 part of the spectrum. 



The Rhodamin B paper was at first prepared by Andkesen (1898). 

 It is a silver bromide paper, sensitized with Rhodamin B. Andresen 

 used it for the estimation of sunlight and Wiesner also has 

 used it for estimations of relative illumination. It is prepared as 

 follows (the description in Wiesner (1907) is wrong): Raw paper 

 for photographic use (for instance 12 kg Rives) is immersed in a 

 solution of 6.1 % KBr for 5 minutes. After drying it is sensitized 

 in a dark room by floating on a solution of 12 % AgN0 3 for 2 

 minutes. Next it is carefully washed out in distilled water to remove 

 the excess of AgN0 3 . At this stage its sensibility to light is small 

 and it is blackened very little by exposure. (The blackening of the 

 unwashed paper is caused by the excess of AgN0 3 in the paper). 

 When the washing out of the paper is finished, it is immersed for 

 5 minutes in a bath, consisting of 200 cm 3 water, 6 g NaN0 2 and 

 12 cm 3 of a solution of Rhodamin B in alcohol (1 : 200) (Andresen 

 uses 8 cm 3 of the Rhodamin solution). By the treatment with 

 NaN0 2 the paper gets its sensibility to light back again; the 

 Rhodamin B makes it sensitive especially to the yellow rays. Care 

 should be taken that the papers do not cover one another in 

 the bath. The prepared paper is red and is blackened quickly by 

 exposure to light. 



In estimations of light intensity it is of great significance to 

 get a good standard colour. Such a colour can, I have found, be 

 made by preparing paper in the manner described above, but 

 without NaN0 2 in the last bath. The appearance of the paper is 



