An optical instrument (10) has a drop-supporting surface for receiving a droplet (98) of liquid with a cover (16) mounted on the housing (12) which receives a light source and provides communication between the light source (114) and the inner surface of the cover. A loading aperture (24) extending through the cover permits access to the drophead when the cover is in a first rotational loading position, and the cover may be rotated to a measurement position in which the light source is positioned to illuminate the drop-supporting surface. A positioning mechanism provided between the cover an the housing engages the cover when it reaches the measurement position and thereby ensures that the light source and drop-supporting surface are maintained in fixed spaced-apart relationship.