Transactions of the Society. 



measured one thing, and I another, and the difference between our 

 measurements amounts to 32 p.c. 



Professor Porter and Mr. Everitt say : — " The question is there- 

 fore not merely one of disagreement between theory and experi- 

 ment, but also one of disagreement between Fraunhofer's and 

 Mr. Nelson's experimental values." 



I have never published nor shown my measurements of the 

 single artificial star to any one, so it is quite impossible for Pro- 

 fessor Porter and Mr. Everitt to know whether they do, or do not, 

 agree with those of the truly renowned Fraunhofer. 



I am still of opinion that the radius of the first dark ring of 



a single star is too large a measure for the telescopic resolving 



limit of a double star; in other words, the resolving limit of a 



5". 555 



telescope is less than ■ . 



a 



This month (October 1907) Mr. W. F. A. Ellison has published 



an account of the separation of &> Leonis and £ Bootis by 8^ inches 



3"*315 

 of aperture ; these results correspond to a limit of and 



3" -06 ,. , 

 respectively. 



These observations are far finer than any I have ever been able 

 to accomplish. Mr. Ellison has thus lowered the value of c to 

 0-6718, and has increased my percentage of 32 to 45. 



The values of the microscopical experiments (Nos. 4 and 13) 

 given in my table may be far from correct. The difficulty of these 

 microscopical antipoint measurements is very great indeed ; those 

 with a telescope are mere child's play in comparison. 



