217 



A NOTE ON THE AMICIAN TEST."" 



By E. M. Nelson, F.R.M.S. 



{Read January 2it/i, 1911.) 



The following letter, which I have received from Mr, F. J. Keeley, 

 of Philadelphia, has reference to a balsam-mounted slide in his 

 possession, labelled : " Navicida Amicii, Florence, Italy. From 

 Prof. Amici to C. A. Spencer." 



A mixed fresh-water gathering, containing a large proportion 

 of typical Navicula rhomhoides, among which are a few valves 

 under 50 microns in length, which form a class by themselves on 

 account of extreme delicacy and transparency, which in the 

 balsam mount renders all but the midrib nearly invisible. On 

 account of this delicacy they are much more difficult to resolve 

 than would be anticipated from the distancing of the markings. 

 They correspond to varieties which have been described as 

 Friistulia saxonica, jYavicula crassinervis. 



A large majority of the valves are fairly robust and, while 

 extremely variable in size, remarkably uniform in the character 

 of the markings, which are readily resolved into dots with central 

 cone under any fairly good objective of r20 to 1"30 N.A. [or an 

 exceptionally well corrected lens of about TOO N.A, E. M, N,], 

 there being scarcely any noticeable difference in this respect 

 between the smaller forms and those of twice the size. As a 

 test they are equivalent to the smaller Xavicula rhomhoides of 

 M oiler's 60 -diatom test plate. 



Measurements of twenty valves follow. These were selected 

 by traversing slide by horizontal movement of mechanical stage 

 and measuring every perfect form that crossed the centre of 

 the field. This method, which I generally follow in selecting 

 objects for measurement, was not altogether effective in the 

 present case, as it resulted in including three out of twenty of 



. * "Navicula o-homhoides and Allied Forms," Journ. Q. M. (7., Ser. 2, 

 Vol. XI., p. 93. 



