Page 111 - Fundamentals of Light Microscopy and Electronic Imaging

94 DIFFRACTION AND SPATIAL RESOLUTION

a b c d e f g h

Specimen Length Period
µ
µ
( m) ( m/stria)
a Gyrosigma balticum 280 0.67
b Navicula lyra 160 1.25
c Stauroneis phenocenteron 150 0.71
d Nitzschia sigma 200 0.43
e Surirella gemma 100 0.50
f Pleurosigma angulatum 150 0.53
g Frustulia rhomboides 50 0.29
h Amphipleura pellucida 80-140 0.27

Figure 6-7
Diatom test plate and key.

Burlington, North Carolina. The accompanying table indicates the cell size and
interstria spacing for the eight species of diatoms on the plate. While performing
this exercise, compare the image of Pleurosigma produced with a modern lens
with that produced by one of the first apochromatic lenses designed by Abbe
(Fig. 6-1). If a test plate is not available, use a grain of dispersed diatomaceous
earth (from a chemical supply house), or, failing that, some household scrubbing
cleaner, which contains diatom shells as an abrasive.

1. Review the steps for adjusting the microscope for Koehler illumination.
Identify and locate the positions of the four aperture planes and the four