Phase contrast microscopy is gradually acquiring an ever widening scope of applicability in the biological sciences. With it much of the intimate structural details of living organisms can be seen without producing the disruptive changes caused by their examination in vacuum with the electron microscope or the fixation and staining required to study them under the compound microscope.
We have been able to study the Treponema pertenue obtained from the primary and secondary lesions of a number of patients with yaws before and during therapy with antibiotics. In this study we have used a Winkel-Zeiss phase contrast microscope with the 90 × condenser phase ring and the 90 × oil immersion phase objective. The light source consisted of a lamp with an adjustable focus and a green monochromatic filter or neutral density filter. We are of the opinion as the result of this study that the darkfield microscope is not only cumbersome but fails to provide the definition of structure of the spirochaete comparable to that obtained with the phase contrast microscope.