Trrn^.Tn^cTiim'] «/ Eigh-j^ower Definition. 31B 



The visibility with a standard power of 1000 diameters may be 

 thus calculated in seconds — 



No of seconds sub- j 20627 X 1000 * 



9 = < tended by a pair oi > = 



lines in a baud j ^o- ^^ divisions to English inch. 



and the corresponding angle in seconds is given in the last column 

 of the above Table. In order now to erect a real standard of de- 

 fining power and tabulate results, the working microscopist having 

 measured by camera lucida the number of lines of beading to the 

 inch of any diatom, has only to divide 20627000*0 by the number 

 of lines per inch, and the result in seconds gives at once a standard 

 of the critical angle of observation at 1000 diameters; when the 

 nearer it approaches 183" the closer will his observation equal the 

 celebrated test of Nobert's XlXth band. In the writer's opinion, 

 some of the finer diatoms, especially the more delicate specimens of 

 Ehomboides, are equal to the XlXth band test, and the beading of 

 which, the central lines or ribs forming the median line being much 

 closer in contact than the XlXth band of Nobert, is one half more 

 severe. 



But all European microscopists will long acknowledge that a 

 world-wide microscopy is deeply indebted to the genius of Herr 

 Nobert for the j)roduction of his exquisite workmanship on glass. 



Mr. Stodder informs us that Dr. Barnard made five counts of the 

 XlXth band with a Spencer xath and a Tolles' ^ih, and that 

 (January 29, 1868) he writes that the observation was made with 

 dry objectives. The mean of the counts are reckoned at 110,820 

 lines to the inch instead of 112,688. 



Dr. Woodward gives the number of lines per millimetre per 

 inch taken from Harting, and Mr. Stodder, the number to the 

 English inch, on the principle that if a thousand and one lines are 

 drawn in a given sj^ace, there are 1000 spaces and 1001 lines; but 

 as it is the separating interval with which we are concerned, from the 

 centre of one line to the centre of the next, I prefer considering each 

 separation the xoWth instead of -^^ for 13and I. 



English microscopists would greatly esteem a statement of Dr. 

 Barnard's own results. We on this side of the water can only fehcitate 

 our American friends upon every advance they can make ahead of 

 us here, even though they surpass us. 



Few observers could equal Mr. Stodder's keenness of sight in 

 clearly and satisfactorily resolving the XlXth band with a power 

 of 550, as it implies a critical angle of 100 seconds, a thing, how- 

 ever, by no means impossible, although it is equivalent to discerning 



* I may add the more correct value of the factor 20G27000 is to be found by 

 dividing unity by the value of a second, sin. 1" = -00000^8481368, and for any 



p 

 other power P and number of divisions N, 6" = 20627 x ^r; or 20627 x P t- N. 



N 



