COSMIC RADIATION — JOHNSON" 



203 



Keferring to figure 2, we assume that an observer can make meas- 

 urements of intensity from any direction and at any point in the 

 lower plane. This plane is analogous to the earth's surface, and it 

 prevents radiation coming up from below. Between the upper and 

 lower planes a magnetic field has the direction of the arrow and is 

 uniform over any section parallel to the front of the diagram, but is 

 of increasing strength toward the back. The line O O" is analogous 

 to a magnetic meridian line in the northern hemisphere of the earth 

 with the point 0" near the Equator where the earth's horizontal field 

 is strongest. Thus the right side of the diagram corresponds to the 

 west. 



FiGDEB 2. — lutuitive illustration of the essential elements in the earth magnetic analysis 



of the cosmic radiation. 



The essential point in the analysis is the realization that the path 

 of a ray in the region of the magnetic field is a circle, the radius of 

 which depends upon the strength of the field and the rigidity of the 

 ray. Rays are incident uniformly from all angles in the upper half 

 of space but the observed distribution in the lower plane is altered. 

 Positive rays of a particular rigidity, corresponding to the curva- 

 ture of the paths represented, and incident from the left horizon 

 reach the station from some angle ^, or from the more inclined 

 angle Q' at 0^ where the stronger field produces more curvature. 

 At the point O" ^ where the field is still stronger, the same radiation 

 may not be able to reach the observer at all. Referring again to 

 station O^ the angular region to the left of Q can be illuminated only 

 by rays of higher rigidity than those which cut off at angle 6*, 



