405 



XI. — Diffraction Rings due to a Circular Aperture. 



By Alfked W. Poetee, B.Sc, Fellow and Assistant Professor of 

 Physics, University College, London ; and P. F. Eveeitt. 



{Bead May I5th, 1907.) 



In a paper by Mr. E. M. Nelson read in March 1906 and printed 

 in the October No. of the Society's Journal, 1906, occurs the fol- 

 lowing sentence in reference to the diffraction rings formed 

 when light from practically a point source is received after passing 

 through a circular aperture : " If the radius of the first dark ring 

 be experimentally determined, it will be found to be 32 p.c. 

 smaller than its theoretical value. What would be thought of the 

 Newtonian theory of gravitation if the calculated distance of Mars 

 from the Sun was 32 p.c. too great i " This somewhat sweeping 

 criticism of the diffraction theory seemed to call for further experi- 

 mental tests before it could be accepted, especially as in the early 

 days of the subject experimental verification of the theory was 

 obtained. It is true that Fraunhofer's measurements were made 

 with white light only, and in that respect are imperfect ; however, 

 when his values for rectangular and circular apertures are compared, 

 the factor of proportionality between the diameters of the first dark 

 ring in the two cases is found to be exactly 1 • 22, as required by the 

 theory. These experimental values are conveniently found quoted 

 in Knockenhauer's Undulations-theorie des Lichtes. According to 

 Mr. Nelson, tliis factor turns out experimentally to be 0*8266. 

 The question is therefore not merely one of disagreement between 

 theory and experiment, but also one of disagreement between 

 Fraunhofer's and Mr. Nelson's experimental values. 



The following measurements were made with the object of 

 obtaining a test possessing as great a precision as possible with the 

 modern means at our disposal. 



The method adopted was that of direct measurement of the 

 diameters of the rings, using a micrometer eyepiece. An artificial 

 star was set up 23 metres away from a telescopic object-glass of 

 good quality of between 2 and 3 inches aperture giving an 

 image at 670 mm. behind the lens ; and diffraction rings of a 

 suitable size were produced by placing a small aperture in front of 

 the telescope. The greatest difficulty was the faintness of the 

 rings when made large. An aperture of 2*23 mm. diameter was 

 found to be the most suitable, and the diameter of the first dark 

 ring was then about 0*2 mm. The artificial star used was a 



