EADIATION" OF THE SUN" — ABBOT. 



161 



perfectly transparent apparatus capable of forming a normal spec- 

 trum. Such a curve is that marked "AH" in figure 5. It is marked 

 "All" because it represents the mixture of rays from all parts of the 

 sun's disk, just as we ordinarily get them. 



Figure 6 shows a number of curves of distribution of rays along a 

 diameter of the sun. From these we see that the sun's disk is not 

 of equal brightness from edge through center to edge, and that the 



95 875 

 9792 825 75 65 55 



PERCENT SOLAR BADIUS 875 95 



to 20 20 40 55 65 75 82g 9? 97 



Fig. 6. 



-Distribution of brightness along the diameter of the solar disk for rays of dufeeent 



wave lengths. 



contrast of brightness is far greater the shorter the wave length con- 

 sidered. In other words, the sun's edge is much dimmer compared 

 to its center for the ordinary photographic plate than for the eye 

 because the plate is most sensitive for violet and ultra-violet rays, 

 while the eye is most sensitive in the yellow. 



Returning again to figure 5, the other curves show the relative 

 intensities of solar energy of aU wave lengths taken from points 



