Abstract
The factors affecting the visibility of fibers under the phase contrast microscope are discussed with reference to (a) the visual acuity of the eye (b) the optical performance of the microscope and (c) the optical properties of the prepared sample. For the first time, it has been possible to measure the limits of detection of small fibers collected on membrane filters. Full experimental methods are presented and the results expressed in terms of phase shifts and minimum detectable brightness differences between the fiber and the mounting medium filter background. Fibers down to about 0.15 µm diameter are visible with good optics provided the phase shift exceeds about 4°. This corresponds to a refractive index difference of about 0.04, and provided this is exceeded further modest changes in mountant refractive index make little difference to fiber count.