^♦5 



PHOTOGRAPHIC INTENSITY OF SUNSHINE. 



A photographic method of determining the brightness of sunshine 

 or sky light is verj' desirable as supplementing the thermometric 

 methods. It is as erroneous to assume that all radiation that falls 

 upon a black-bulb thermometer is absorbed by it and converted into 

 heat and measured by the expansion of the mercury as it is to assume 

 that all the radiation that falls on a photographic film is absorbed 

 by it and is represented by the chemical changes that take place in the 

 film. Equally erroneous Avould it be to assume that all the radiation 

 that enters the eye is represented by the impression of brightness 

 conveyed by the retina to the brain. In order to measure in absolute 

 units the total energy radiated from the sun, we need a proper 

 summation of the thermal, visual, and photographic work done by 

 the radiation. If we wish to determine only the intensity of that 

 part of the radiation that does the Avork in which agriculture is 

 chiefly interested we should consider only the heating effects of the 

 radiation and the special chemical effects manifested in the action of 

 sunlight upon chlorophyll. 



The action of the sunlight upon the chlorides and bromides of 

 silver, as in ordinary photographic processes, may not be an exact 

 measure of its action upon the leaves of plants. Some other chemicals 

 may be more appropriate for use at agricultural experiment stations, 

 but the photographic methods perfected by Profs. H. AV. Vogel and 

 L. Weber are worthy of trial as a first step in the right direction. 

 These processes give us the relative intensity of the radiations that 

 belong to the blue end of the spectrum, with only a small admixture 

 of the influence of green and yelloAv rays. 



During the year 1890, as the result of a numerous series of observa- 

 tions at Kiel, Prof. L. Weber found that the reddish light of Jjie 

 spectrum on dark winter days has only about 500 times greater int4M4^- 

 sity than the quantity of light from a normal candle at a distance of 

 1 meter, when measured by their relative effects on a photographic 

 plate, while at the same time the photographic intensity of the green 

 light of the spectrum was four times as much. On bright sunnner 

 days the intensity of the red light was 50,000 times that of the candle 

 at 1 meter, while the intensity of the green light was about 200,000, 

 or about 4 times as much in summer as in winter. The intensity of 

 (he blue light in the solar spectrum was about 25 times that of the 

 red light, which ratio varied a little with the kind and amount of 

 cloud. In all this photographic work a very sensitive silver bromide 

 paper was used; so that these results, strictly speaking, relate only to 

 the variations in the intensity of those special rays that affect this 

 chemical. But these variations will be nearly parallel to the diurnal 

 and annual variations of the rays that affect the growth of plants. 



