Abstract: A portable testing device for examining biological samples in one embodiment comprises a support and a tubular member having a lens holder adjacent one end thereof rotatably mounted on the support. A light source is moveable axially within the tubular member between an on position where the light source is illuminated and an off position. A biological sample support surface is provided adjacent the lens holder on which a biological sample to be examined is placed. A lens through which a user can look to examine a biological sample placed on the biological sample support surface is removably supported by the lens holder. The light source and tubular member carry co-operating formations which are configured such that the light source moves axially within the tubular member upon rotation of the tubular member. In another embodiment the light source is stationary and is responsive to a light-sensitive circuit.