177 



The beading of the Pleurosigma formosum, one of the 

 coarsest of the diomataceoe, and which varies from 32,000 to 

 36,000 beads to the inch, cannot well be distinguished with 

 a less power than 250 linear ; below this, striae make their 

 appearance, of a cylindrical character. 



Here = 20-628 X ^^ = 20628 -^ 

 00 36 



= 143" 

 = 2'„23" 



Perhaps very acute vision may detect them with a power 

 of 174"5, in which case = 100" very nearly — 



For Q = 20-628 X ^^ =100" nearly. 



But at ten inches' distance a bead of this size really presents 

 a visual angle of about 4- a second (0"'572). 



Assuming, therefore, that 1' is the smallest angle at Avhich 

 beading can be conveniently defined as such, unless they are 

 represented Avith central black shadows, and that one tenth 

 of this, or 6", is even a limit of sphericular visibility, a table 

 may be conveniently employed as a means of testing the 

 aberration of objectives as follows : 



Diameters of Beading. 



But practically it is found that a power of 1200 most con- 

 veniently shows this beading, and even this fails to distinguish 

 the structure appearing in the spaces separating the beading. 

 The results would then take the following form : 



We conclude from these tables that it is rather superb 

 definition than great amplification which should render 

 minute structure visible. A power of 12,000 applied to the 

 Formosum displays the beading, indeed, of large diameter, 

 but with an ordinary y^ objective the aberration receives a 

 proportionate exaggeration. 



It should be observed that the third row is finer than 



