1897.] 



near the Focus of a Telescope. 

 Table II. 



269 



(2) On the marks made by stars on photographic plates ex- 

 posed near the focus of a telescope. By H. F. Newall, M.A., 

 Trinity College. 



The appearances presented by some photographs of stars 

 taken with the 25-inch visual refractor of the Cambridge Ob- 

 servatory in the years 1893-4 were described and illustrated in 

 a paper " On the formation of photographic star-discs " by the 

 present writer, in the Monthly Notices of the Royal Astronomical 

 Society, Vol. liv. p. 515. In that paper attention was called to 

 the existence of a considerable concentration of light in the 

 boundary of the circle of diffusion for light of any wave-length ; 

 and I was led by a series of observations and experiments, which 

 gave much information as to the excellent performance of the 

 object-glass, to express the view that "it may be a straightforward 

 result in the theory of diffraction through a circular aperture," 

 that this annular concentration must exist. 



The extension of Lommel's work (Bavar. Acad. Mem. 1886, 

 Vol. XV., p. 235), which Mr Mayall was good enough to undertake 

 at my suggestion and has accomplished and described in the 

 preceding paper (page 259), shows that near the focus of a perfect 

 object-glass with circular aperture the circles of diffusion must 

 exhibit an intensification of light of any given wave-length near 

 the boundary. Mr Mayall has in fact found that the undulatory 

 theory accounts for just such an effect as my experimental results 

 led me to anticipate. 



A few more observations, and also some photographs taken 

 since 1894, have only confirmed my view that whilst there are 

 signs of some slight outstanding spherical aberration in the 

 25-inch refractor, the main factor in the production of the rings 

 which I have measured in the star-marks obtained with special 

 exposures of photographic plates has been the concentration of 

 light near the boundaries of the circles of diffusion for different 

 colours. 



21—2 



