18 CORA D. REEVES 



mm. wide, measured on a line parallel to the Nernst fila- 

 ment and to the width of the slit. 



Assume the conditions shown in figure 8 and let W equal 

 the width of the slit. To find the intensity of illumination 

 at 0, let z be the range of points on the light-source from 

 which light reaches 0, and let d be the distance from such 

 points to 0. Also let @ be the angle which d makes with 

 the line perpendicular to the plane of the lens at 0. Then 

 the intensity of illumination received at from each point 

 on the source is: 



K Cos2 6) 



Intensity received at O from each point on the source = 



d^ 

 d2 = 218- f z2 



2182 

 Cos^© = 



%* the total intensity at 0= K 



218Mz 



(2182 + z2)2 

 109w 



Similarly, let d' be the variable distance from points on 

 the light source to P; let /3 be the angle which d' makes 

 with the perpendicular from the light source to the line 

 PQ; and let 7 be the angle which d' makes with the line 

 perpendicular to the plane of the lens at P; then the in- 

 tensity at P from a point in filament equals: 



1 

 K X X cos/i X cosx 



(di)2 



Perpendicular from filament to PQ 



= \/ (218)2 + 1600 



di = \/(40 +z)2 + (218)2 + 1600 

 \/(218)2 + 1600 



cos 5 = ■ 



\/(40 +z)2 + (218)2 + 1600 

 218 



cos;' = — . — 



\/(40+z)2T(2r8y2 + 1600 



