©Lin Yangchen

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Imagine a landscape of deep canyons snaking across the earth’s crust, punctuated by gigantic sinkholes with vertical precipices dropping thousands of feet to the bottom. It sounds like an expedition to Mexico or Mulu, but one needs voyage no farther than one’s stamp album.

You could mistake this for a satellite pass over the fractured karst landscape of the Yucatán Peninsula, with its ancient and mysterious cenotes concealed in lush forest. The main image is one of 23 optical sections of a fluorescence-mode confocal laser scan of the chalky surface of the upper-left fascia pattern of a coconut definitive. The 3.60 μm-thick section was visualized by combining the fluorescence signals from simultaneous laser excitation wavelengths of 404.8 nm (1.7 mW), 486.2 nm (0.9 mW), 561.5 nm (0.9 mW) and 638.8 nm (1.6 mW). Laser power was measured at the tip of the optical fibre and totalled 5.1 mW before passing through the objective on its way to the stamp. The dwell time was 3.03 μs and there was no visible damage to the stamp. The fluorescence signals were amplified through gallium-arsenide-phosphide photomultiplier vacuum tubes. The chalky surface is both pitted and cracked. The deep orange spots in the image are particles of unknown composition fluorescing at approximately 595 nm. These are not discernible under normal illumination. The bars at the bottom and right side of the figure show the full z-profiles of the fluorescence along the x and y crosshairs. The resolution on the x-y plane is 0.78 μm, while the z-resolution is 10.26 μm.

A snowy Christmas

The Straits Settlements $5 top denomination on green paper.

Micrograph in the spirit of the post-war American art movement of Abstract Expressionism, as characterized by quasi-spontaneous blots and brush strokes. Nikon Ni-E microscope, 20× plan apochromatic objective, Nikon DS-Ri2 35-mm-format full-frame camera, extended depth of focus with step size of 0.9 μm.

A particularly bad case of coconut pox. Perhaps the liquid coating mixture was overheated or too thick.

Some of the largest craters ever seen on the coconut definitive, measuring about 70 μm across. Deposits of the blue pigment used to colour the paper can be seen embedded in the crater floor. The translucent rim raises suspicion of overhangs arising from bubbles in "prehistoric" times when the "crust" was still "molten".

Confocal laser scan of a pit 50 μm deep. Field of view about 250 μm each side. Optical sections 0.50 μm thick, pinhole 1.2 airy units, pixel dwell 1.09 μs, horizontal resolution 0.28 μm, vertical resolution 1.44 μm.

The simulated underside shows the cracks reaching depths of about 30 μm. The confocal microscope was able to resolve the three-dimensional morphology of these narrow cracks where the conventional optical microscope failed. The pinhole in the confocal system eliminated out-of-focus light that would have "polluted" the in-focus light from inside the crack. Signal fidelity is critical here because of the extreme z-gradient.

A 3.0 kV scanning electron micrograph (upper image) of a pit at 1000×, and the 8-km-wide caldera of Tambora volcano (lower image: nasa), where the most powerful eruption in recorded history took place in 1815. The author summited the volcano in 2006. Now he has his own miniature Tambora at home, complete with volcanic fissures, lava flows and fumaroles—and Gustav Mahler's Symphony No. 6.

Altum foraminis (“deep hole”)
in the spirit of lunar exploration
Red-cyan anaglyph created from a stereo pair of scanning electron micrographs with the stage tilted at −5° and 5°. As the microscope projects the image like a telephoto lens, the simple translational movement used in conventional stereo photography does not give a sufficient perspective difference, especially of the sharp and overhanging edges. The relatively large angle of tilt also simulates a close-up view, like that from a helicopter hovering over the sinkhole, its intricately sculpted walls reminiscent of limestone carved and scalloped over millions of years by a subterranean river.

The author's sketch of a scanning electron microscopy setup for looking under the overhanging lip of the pit. The plan was eventually abandoned; it required prolonged electron bombardment that would overcharge the specimen and blow out the image.

I am grateful to David Beech, Benedict Sim, Ernest Cheah, Clement Khaw, Wulf Hofbauer, Li Zhen and Goh Wah Ing for discussions and technical assistance. I also thank the Nikon Imaging Centre and Science Centre Singapore for the microscopy facilities that made this research possible.


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