310 On a New Critical Standard Measure 



TMonthly Microscopical 

 L Jouriuil, June 1, 1»70. 



May, 1869,* in this Jonrnal, that the hues of Nohert's bands, 

 commencing with the Vlllth baud, are separated under the micro- 

 scopic power of 1000 diameters by the following critical angles 

 (carefully verified) : — 



Table of Standard Ckitical Angles subtended by a pair of Nobert's 



Lines. 



The performance of American and English glasses, as recorded 

 in the papers already alluded to, may be conveniently referred to 

 this standard as follows : — In the last column I have given the cri- 

 tical angle of resolution, after reducing the given power to a uniform 

 standard of 1000 diameters. In each case the B eye-piece seems to 

 have been used, and the recorded experiments embrace objectives 

 from the ^th, yVtli to yVth, rrth, to the oVth focal length. 



Nobert has selected the Paris line, which equals TT/WVVWths of 

 the Enghsh inch, as the basis of his scale. In the first band this 

 is divided at the rate of 1001 lines into 1000 spaces, and each hne 

 is TsWth of a Paris line apart. 



The 2nd band contains lines TTootl^ apart. 

 ») 3rd „ „ TooTJ'''^ » 



„ lotll „ „ 8 5 00 1)'''^ )> 



1 Qth 1 th 



Eecalculating these dimensions, and verifying the results care- 

 fully, the lines are separated by the following parts, conveniently 

 represented by the number of divisions to the inch : — 



The first column represents the Angulata of various degrees of 

 fineness, and the second column the Navicula Ehomboides and 

 Amphipleura pellucida, which, however, in many specimens, are 

 finer still. 



"■ ' Monthly Microscopical Journal,' Dec, 1869. 



