A transparent plastic slide has a recessed chamber which receives a transparent plastic cover. The cover has a fluid inlet opening which overlies a recessed center well in the slide. The well has a sloping ramp which extends to a viewing platform which has a surface which is closely spaced from the underside of the cover to define a constant thickness viewing cavity surrounding the well. An overflow channel surrounds the viewing platform. A vent hole extends through the cover above the overflow channel. The sample is introduced into the center well through the fluid inlet opening. The fluid flows into the viewing cavities by capillary action, and is free to overflow the viewing cavities and be received within the overflow channel, thus reducing boundary effects. An inlet port may be provided above the viewing platform to permit substances to be added to a sample while it is under observation.