Monthly Mlcroscojtlcan 

 Journal, Juue 1, 1S70. J 



of Iligli-itoiver Definition. 



309 



the museum, May 16tb, from Mr. Cliarles Stodder, wlio stated in 

 his accompanying letter that it might be regarded as a fair sample 

 of Mr. Tolles' work. With this lens I was unable to see the true 

 lines beyond the 16th band." 



" It will be seen, then, that in my hands the best definition was 

 obtained by the immersion xVth of Messrs. Powell and Lealand." 



" A careful count of the lines gave the following results : — 



" If Mr. Mayall .... has not been able to see the lines in 

 Nobert's plate as distinctly as they are shown in the photographs 

 submitted (to the Eoyal Microscopical Society), I must presume 

 simply that he has not illuminated the object with monochromatic 

 light." 



Mr. Mayall, however, omits to state in his paper that he has 

 employed Powell and Lealand's xVth on the XlXth band, made on 

 the immersion principle. 



The rival claims of Mr. Stodder and of Colonel Dr. Woodward to 

 have fairly resolved the XlXth band will be well understood by read- 

 ing the papers themselves. The congratulations of the microscopista 

 of this country, however, may be honestly conveyed to Dr. Woodward 

 upon his successful photograph of those difficult lines by means of 

 an English objective. 



I may now be allowed to state that, after a careful analysis 

 of the experiments detailed in those papers, the following results 

 have been arrived at, which are given in a tabulated form. The 

 convenient method of judging of the dividing power of telescopes by 

 means of close double stars is strongly recommended by the writer 

 to be adopted as a new standard of definition for the microscope 

 as in the telescope. 160" is the visual angle subtended by the 

 double 7^ Andromedae, separated (centre to centre) by 0"*4 under 

 a power of 400 diameters. This is a smaller angle than any presented 

 by a pair of lines of the XlXth band counted at 1 000 diameters. But 

 in order to make the calculations sure, the following Table has been 

 carefully computed, beginning with Nobert's Vlllth band, and 

 ending with the XlXth. The principle selected is this : the last 

 line is left out just as in dividing an inch into ten parts we say 

 there are ten lines to the inch, although reckoning each terminal 

 line there are eleven. 



Taking, therefore, the usual approximate value for one second 

 = • 000004848, and for the sake of estabhshing a standard critical 

 angle for the testing of the performance of the microscope, it will 

 readily be calculated, on the principle laid down in my paper of 



