42 BULLETIN 1059 1 , U. S. DEPARTMENT OF AGRICULTURE. 



length, it is necessary to consider the constantly changing absorption 

 by the earth's atmosphere. According to Very (75), who cites Lang- 

 ley (Professional Papers of the Signal Service, No. 15), there are 

 ■" two different kinds of selective depletion which the solar rays suffer 

 in traversing the earth's atmosphere. One kind is greatest for the 

 rays of shorter wave length, and diminishes by perfectly regular 

 gradations as one passes toward the longer waves of the infra-red. 

 Its cause may be referred to selective reflection or diffraction of the 

 shorter ether-waves by particles of excessive minuteness. The other 

 Tdnd of absorption produces irregular gaps or depressions in the 

 spectral energy curve, which begin at the red end of the visible 

 spectrum and grow in magnitude and frequency as the wave length 

 increases. Researches by Abney and Festing, and by other investi- 

 gators, have traced the majority of these depressions to the action of 

 aqueous vapor." In the extreme infra-red there is shown to be al- 

 most total absorption from this source. 



The light, principally of the shorter wave lengths, which is dif- 

 fused by minute particles in the atmosphere, is not entirely lost, but 

 may be measured as skylight, probably of greater wave length than 

 the original direct rays. The infra-red rays which are so greatly 

 absorbed by the vapor of the atmosphere, merely heat the upper at- 

 mosphere, and to this extent, of course, are lost as solar radiation. 



2. Looking at the matter from another viewpoint, and accounting 

 for the rather regular daily change in sunlight intensity at a given 

 point on the earth's surface, Kimball (63) after showing the greater 

 intensity of all wave lengths at midday when the light passes through 

 minimum thickness of atmosphere, makes interesting comparisons 

 of the total and luminous radiation under various circumstances. 

 Radiation from an overcast sky is slightly richer, and radiation from 

 a clear sky markedly richer, in luminous rays, than is direct sunlight. 

 Direct sunlight decreases in luminous richness as the sun approaches 

 tfye horizon. 



3. These few facts point to the uselessness of photometric methods, 

 depending on the chemical action of rays of rather limited wave 

 length, to measure the total radiation or any part of the radiation 

 other than the few wave lengths which may be involved in the par- 

 ticular reaction. Thus, for example, even if it were assumed that 

 silver chloride was decomposed in proportion to the intensity of a 

 given section of the spectrum, a certain reaction with silver chloride 

 might be secured with other wave lengths varying through a very 

 wide range. 



Again quoting Very (75), it is seen that photochemical processes 

 are very complex and hazardous as a measure of energy : 



While luminous effects may be regarded as dependent on a certain photo- 

 chemical action upon the retina, not all photochemical processes are equally 



