MICROSCOPY. 813 
With the ammoni-sulphate cell it is instantly and clearly 
shown covered in every part with squares, like Pleurosigma 
Balticum. 
Navicula crassinervis.—The specimens of this variety, in my 
possession, are more difficult than Frustulia Saxonica and even 
rival A. pellucida under lamp illumination, but any clean frustule 
is satisfactorily resolved. 
sing monochromatic light with plain mirror and Wenham’s 
paraboloid, longitudinal lines are discovered. After careful manip- 
ulation both sets of lines are seen at the same time, and an ap- 
pearance of beading results. 
Navicula cuspidata.— Both sets of lines are easy, but the longi- 
tudinal are much closer together than the transverse. _Conse- 
quently the light interlinear spaces are elongated and no semblance 
of beading is to be seen. In diatoms where the intersecting striæ 
are of nearly equal fineness the little square spaces, when not well 
defined, seem circular, and if the illumination by transmitted light 
is intense they present a raised appearance due to refraction. 
Mr. Charles Stodder called my attention to this diatom with the 
view of ascertaining with the 5 whether or not the two sets of 
lines lie in different focal planes. My observations many tims 
repeated have convinced me that they are never both in focus at 
the same time, and further that the longitudinal lines are on the 
external surface and the transverse on the internal plate. If there 
are not two plates the lines may be on opposite surfaces of the 
same plate. 
White cloud illumination is found to be much better than other 
and more brilliant light for demonstrating these slight differences 
in focal distances. Many errors of interpretation are avoided by 
using an approximately central pencil when the instrument used 
is capable of elucidating all the details of structure without 
greater obliquity. 
Frustulia Saxonica.—In addition to my observation of longitudi- 
nal lines upon this test and resolution into dots (NATURALIST, 
July 1873, page 443) it may be worth noting that even with lamp 
illumination the ;1, has displayed the transverse much clearer than 
they appear in Dr. Woodward’s photo-print. (Lens, vol.i, p. 197.) 
With oblique light direct from a small German student’s lamp, 
without mirror, prism or condenser of any kind, a person entirely 
unaccustomed to the microscope could distinctly see them with 
