

JOURNAL 



OF THE 



ft 



ROYAL MICROSCOPICAL SOCIETY. 



FEBRUARY, 1908. 



TRANSACTIONS OF THE SOCIETY. 



I. — A Reply to Professor Porter's mid Mr. Everitt's Criticism 

 upon my Paper on the Resolving Power, etc. 



By Edward M. Nelson. 



(Bead November 20, 1907.) 



Duking the recess Messrs. Porter's and Everitt's paper, criticising 

 my limits for the resolving power of a telescope, has been published 

 in the Journal. In the meantime, I have gone over the experi- 

 ments a second time, and results substantially the same as those 

 printed in my paper have been obtained. These experiments 

 prove that the constant (called c in my paper) is 32 p.c. less than 

 the radius of the first dark ring, as calculated by Airy, and as 

 measured, formerly by Fraunhofer and Cooke, and now by Pro- 

 fessor Porter and Mr. Everitt. 



The measurements published by Professor Porter and Mr. 

 Everitt are those relating to the rings, etc., seen in a telescope 

 when pointed to a single artificial star ; but the measurements 

 published in my paper are those made with artificial double 

 stars, when the separation in the telescope was a minimum visible. 

 The following quotation from my paper * shows that this is so : — 



"The first dark ring No. 2 was measured by removing an 

 artificial double star from a telescope until the dark rings made 

 contact ; the distance of the telescope from the star was then 

 measured, and the separation of the stars being known, the angle 

 was found." 



The " No. 2 " refers to the number opposite the experimental 

 result f which Professor Porter and Mr. Everitt have selected 

 for criticism. In brief, Professor Porter and Mr. Everitt have 



* See this Journal, 1906, at foot of p. 524. t Toin. cit., table on p. 525. 



Feb. 19th, 1908 b 



