RELATION OF LIGHT-INTENSITV TO ADVANCE GROWTH. 151 



specific number. In each group a Watkins Bee Meter was 

 exposed until the sensitised paper had assumed the normal tint, 

 when the time of exposure was recorded. Three such readings 

 were made in different parts of each group and the average was 

 ascertained. In a similar manner, and immediately after taking 

 the last reading, three readings were made in the open in full 

 daylight, and the average was again obtained. The ratio of the 

 reciprocals of the average readings was then calculated for each 

 group, and this gave the specific photic ration, which is here used 

 as a measure of the light-intensity. The advantage of this 

 method is at once evident, for, had the specific photic ration not 

 been employed, it would have been necessary to take readings 

 of the light-intensity over the twenty-one groups at the same 

 instant. It should be mentioned here that these observations 

 were carried out while the trees were in full leaf, also, owing to the 

 exceptional amount of sunshine in the summer of 191 1, the 

 readings were taken in all cases with practically a cloudless sky. 



According to Schimper, the above method of Bunsen and 

 Roscoe for measuring the intensity of light in some degree 

 satisfies the demands of exact research, although it is applicable 

 only to the so-called chemical portion of the spectrum, i.e. to 

 the blue, violet and ultra-violet rays. He states that " 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 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, // seconds,. 



the intensity of the light is i divided by 2, 3, 4, 5, n 



respectively." 



" 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 rattofi.^ The ratio between 



the 



e two intensities (y ) is the relative or specific photic ration Z."- 



1 Photic ration is the equivalent of Wiesner's term Lichtgenuss. 

 - See Plant- Geography upon a Physiological Basis, pp. 55, 56. 



