46 OF THE CHARACTERISTICS OF THE DIATOMACEJE. 
the higher powers, Pleurosigma angulatum is a fair object. 
In the former of these two, with direct light, longitudinal 
strise are seen; but, when the rays are thrown on it obliquely, 
a second series of lines appears at right angles to the longer 
axis of the valve. A threefold system of striation is dis- 
covered in Pleurosigma angulatum by careful adjustment 
of the light, viz. a set of lines perpendicular to the vitta, 
and two other sets of lines running diagonally across the 
valve in opposite directions. 
The student will find on several of the plates examples 
of 1 striation.’ I would refer him especially to Plates n., v. 
(25, 26), vm. (38), xi. (49). 
I need scarcely add that, in such delicate operations, 
where the slightest displacement of the focus changes the 
whole aspect of the picture, a steady hand and a sharp eye 
are indispensable. 
As the number and direction of the strise are variable 
in the different species, though singularly uniform in in- 
dividuals of the same species, they are most important 
elements in grouping these plants. For this purpose the 
lines must be counted, an operation requiring no little 
clearness of vision; indeed, it is well to give the eye a 
thorough rest for some seconds before beginning. As a 
rule, striations showing forty rows or less in 1*1000 inch, 
are managed without difficulty where the illumination is 
rightly attended to, and the defining power of the objective 
is good.* The case, however, is different with the finer 
markings, and about these there seems to be the greatest 
would be given by multiplying that in one direction by that in the 
other, or by itself ; i. e. squaring the linear magnifying power. 
This is called the superficial measurement. Thus supposing a mi- 
croscope to magnify 40 diameters, 40 x 40 = 1600 would express the 
magnifying power in superficial measure.’ (Micrographical Diet, 
p. 448.) 
* Of the two species of Pleurosigma mentioned above, P. attenua- 
tum has 30 longitudinal and 40 transverse strise ; P. angulatum, 52 
diagonal strise in 1*1000. 
