Infrared video microscopy for the study of isolated graptolites

Portal of Graptolite and Pterobranch Hemichordates

INFRARED VIDEO MICROSCOPY FOR THE STUDY OF ISOLATED GRAPTOLITES
Michael J. Melchin, Peter Berridge and Alan J. Anderson
Graptolite News 1995, 8, 49

The fact that graptolites, and many other organic-walled fossils that are opaque to visible light, may be transparent to
infrared radiation has been known for many decades. However, infrared photomicrography has not gained widespread use
among graptolite specialists, presumably as a result of the difficulties in obtaining satisfactory images that revealed
structures of interests. The growing importance of fusellar growth bands and internal structures as they relate to astogenetic
patterns and colony growth has lead to the search for a new way of using IR radiation that reveals these features
non-destructively, in as wide a variety of specimens as possible.

Infrared video microscopy (IVM) uses a relatively inexpensive video camera that is sensitive through the visible and near IR
range, up to wavelengths of 1300 nm. This camera can be connected to a normal biological or petrographic microscope.
The best results are obtained when visible light is filtered with a 1000 nm, longpass filter, although the crossed polarizers on
a petrographic microscope can serve a similar function. The images obtained can be viewed on a video monitor, printed by
direct connection to a video printer or else electronically stored using a computer video image capture system. The stored
images can then be analyzed using image analyses software, printed or photographed through a slide-making system.

Provided the specimens are not of high thermal alteration or very thickly covered in cortical tissues and are not covered or
infilled with IR-opaque mineral material, the resulting images reveal surface morphology, fusellar banding (and occasionally
cortical bandages) as well as all internal walls and structures. Optimum image quality requires that the specimens be
immersed in a very high refractive index liquid, preferably 1.76, the refractive index of the graptolite periderm material. The
best visualization of internal structures can be achieved using stereo-pairs of prints of the video images. These can be
obtained by using a universal stage and tilting the specimens ca. 8 degrees between prints.

GRAPTOLITE NET
is edited and periodically updated by
Dr Piotr Mierzejewski, Count of Calmont
since 2002