SOLAR RADIATION ABBOT 



117 



spots due to the outrush of gases from within the sun and their 

 consequent cooling by expansion as they reach elevations of dimin- 

 ished pressure. If in either of these ways local irregularities occur 

 in the sun's surface brightness, the complete solar rotation which 

 takes place in about four weeks must present these fluctuating in- 

 tensities toward the earth in their turns, and so produce short-inter- 

 val variations of the solar constant of radiation. 



There is another variation of solar radiation, not periodic, but ex- 

 ceptionally interesting to theorists, as it throws light on the sun's 

 inner nature. I refer to the difference of brightness between the 

 edge and the center of the solar disk. Figure 6 illustrates this obser- 

 vation and shows how different is the phenomenon when viewed in 

 differently colored rays. In the ultra-violet, the sun's center appears 

 about three times as bright as its edge, while in the infra-red the edge 



Brichtness Distribution Alons Sun's Diametek 

 Fob Different Colors 



Infra-red 

 ;»= 1.5 5/^ 



iNrHA-RtO 



Xn.966yU 



LTltha-violct 



FlQORB 6.- 



-Contrast of brightness between center and edge of the sun's disk as seen in 

 different colors. From Smithsonian observations 



is almost as bright as the center. A great deal of theoretical investi- 

 gation has been based on exact measurements of these phenomena, 

 which have been carried out in the years 1913 to 1920 by Smith- 

 sonian observers on Mount Wilson, Calif. This study is also asso- 

 ciated with the determination of the distribution of brightness as 

 between different wave lengths in the solar spectrum. Smithsonian 

 observers have determined this as shown by Figure 7. 



The study of the dependence of plant growth on radiation has 

 lately been taken up by the new division of radiation and organisms 

 at the Smithsonian Institution at Washington. In order to have a 

 better control of radiation intensity and hours of exposure than clouds 

 and night would permit if solar rays alone were employed, we use 

 mainly electrical lamps of special construction; the results will be 

 applicable to the understanding of plant growth under natural 

 conditions. 



The essence of the problem lies in this, that plants grow by taking 

 in carbonic-acid gas from the air through millions of little mouths 

 called stomata which dot the under surfaces of the leaves. But this 

 feeding occurs only when certain rays found in sunlight and other 

 sources shine upon the plants. The questions are: Which are the 



