332 PROFESSOR L. BECKER ON THE DISTRIBUTION OF 



I cannot finish this paper without expressing my indebtedness to the University 

 Court of Glasgow for a grant of 100 towards the expenses of the expedition ; to my 

 companion, Mr. JOHN FRANKLIN ADAMS, who presented half of this sum to the 

 Court and superintended the arrangements for the transport of the instruments ; to 

 Mr. ANDREW CROOKSTON, Glasgow, for his hospitality at his comfortable house at 

 Kalaa and the help the employees of his firm extended to the expedition en route ; to 

 the Council of the Royal Dublin Society for the loan of a siderostat, and to my 

 companion, Mr. HENRY A. MAVOR, M.Inst.C.E., Glasgow, who, in the capacity of 

 physician, engineer, and adviser, took upon himself much of that work which is not 

 mentioned, but is so important to the success of an expedition. 



APPENDIX I* 



Diffraction due to the Screens. 



For the first four exposures, each of about a second, the aperture of the lens is 

 reduced by a perforated screen which has thirteen equal circular openings. The 

 arrangement of these openings will be seen in fig. 1 : there are six holes in the 

 corners of a regular hexagon, one in the centre, and six others are equidistant from 

 each two of them. The diffraction pattern of a star does not consist, as might be 

 thought, of a series of detached images which lie on lines intersecting in a centre, 

 but, as photographs of a Lyrse have proved, shows, apart from a central region, 

 luminous rings at the same distances at which one opening produces them. On the 

 photograph of a Lyrae rings are visible as far as 5n (linear value of TT = 9) for the first 

 screen, and on the eclipse photographs the prominences have certainly made no 

 impression beyond lOn-. Faint though the intensity of the rings be, it requires 

 investigation whether a distant ring belonging to a point of the corona near the sun 

 has an intensity comparable to that of a distant point on whose image the ring is 

 superposed, or rather whether all the diffracted light together is not a negligible 

 quantity. I shall show that it is small. The result would have been different if the 

 exposures had been longer and more distant parts of the corona had been photographed 

 with the screens. 



Let P be a point in the focal plane and C the position of its central image. 

 I introduce a rectangular system of co-ordinates XY in the plane of the screen, origin 

 in centre of the central opening, and X-axis parallel to CP. 



Let there be only two holes which lie diametrically opposite and whose centres 

 have the x-co-ordinates x, then the state of oscillation at P is given by 



o 



kirp 3 - Jj (u) 2 cos (rx) sin a, 



w 



* Postscript, added at the request of one of the Referees. The photographic experiments were 

 subsequent to and confirmatory of the mathematical analysis. 



