8 Mr. A. A. Michelson on the Application of 



Case III. — Illumination not Uniform. 



On applying the preceding formulae to observations on 

 images of the sun's disk the angles obtained were all too small 

 by about 10 per cent., the cause of the discrepancy being the 

 want of uniformity of illumination. 



The curve (fig. 4) represents the intensity at any point of 

 the sun's disk as a function of its distance from the centre, 



The ordinates are taken from Yogel's Table *. 



Fig. 4. 



Rff. 5. 



From this curve we obtain that shown in fig. 5 by inte- 

 grating the expression 



•/o 



Fig. 6. 





(9) 



This value of §(x) substituted in (1) gives on integration 

 * Young on « The Sun.' 



