LIGHT 55 



2. PHOTOMETRIC METHODS. 



The methods for measuring the intensity of light are far less thorough 

 than those that serve to determine the temperature and the humidity 

 of the air. Bunsen and Roscoe have succeeded in inventing a method 

 that in some degree satisfies the demands of exact research ; but it is 

 applicable only to the so-called chemical portion of the spectrum, that 

 is to say, to the blue, violet, and ultra-violet rays. The method is as 

 follows : — A photographic paper prepared in a certain way, and termed 

 normal paper, is exposed to the action of light and the resulting discoloura- 

 tion is compared with a constant shade of colour, normal black, at the 

 same time the period of exposure is noted. Bunsen and Roscoe have 

 clearly proved that in a normal paper changing colour in the presence 

 of light, when a definite shade of colour is attained the product of the 

 light-intensity multiplied by the period of time of exposure is always 

 the same. As unit of measurement of the chemical intensity of light, 

 a darkening of the normal paper is selected agreeing with normal black 

 and produced in one second. If the shade of normal black is produced 

 on the normal paper in 2, 3, 4, 5, ... n seconds, the intensity of the light 

 is 1 divided by 2, 3, 4, 5, ... n respectively l . 



The Roscoe-Bunsen method has been further improved and essentially 

 modified by Wiesner, for use in determining the quantity of light available 

 to plants. In its original form, the method is suitable for measuring only 

 weak intensities of light, whilst the determination of higher intensities is 

 faulty, owing to the excessively rapid assumption of the normal tint. To 

 remedy this defect, Wiesner used a scale of several carefully graduated 

 shades of fast colour. 



Wiesner's brilliant investigations were in the first place concerned with 

 the ratio between the intensity, i, of the light actually falling upon a plant 

 or its parts or its habitat, and the intensity, I, of full daylight at the same 

 time. The intensity, i, is the absolute photic ration 2 . The ratio between 



the two intensities, -, is the relative or specific photic ration, L. If, for 



instance, i = -252 and I = -756, then - = — $— = L = -. When the absolute 



1 756 3 



photic ration approximates to the intensity of full daylight, if for instance 



L = — or -, then, whether the daylight be feeble or intense, the absolute 

 1.5 2' 



photic ration, i, varies directly with the intensity of full daylight, I, and the 



relative photic ration, L, remains constant or nearly so. On the other hand, 



when the absolute photic ration, i, is far below the intensity of full daylight, 



I, then the relative photic ration, L, has daily fluctuations as the daylight 



1 Wiesner, V, pp. 301-2. 



2 Photic ration is the equivalent of Wiener's term Lichtgenuss. 



