A cartridge comprising: a body of non-magnetic inert material adapted to be slidably engaged in a receptacle of an apparatus for performing optical analysis of a fluid sample, said body having a domed cross section transverse to a longitudinal axis with an optically clear flat land portion adapted to be positioned in a field of observation for optical analysis; a test chamber underlying said flat land portion adapted to be filled vertically along the longitudinal axis with said fluid sample; and a vestibule having an enlarged mouth at an entrance end and communicating with the test chamber through an inlet opening opposite the entrance end, said vestibule being disposed above said test chamber when the cartridge is vertical and adapted to act as an overflow reservoir capable of being sealed both from said test chamber and an exterior environment, when a stopper is provided for sealing the vestibule at both the entrance end and the inlet opening, said body further comprising a handle portion comprising two lateral sides with grips (37, 38) and an uppermost side with an opening for receiving the stopper, said handle portion having an inner framed structure that is bounded laterally by two keeper elements (77, 77) for keeping the stopper in place and along the longitudinal axis by said uppermost portion and by the enlarged mouth of said vestibule, said cartridge being characterised in that: a. the lateral sides of the handle portion flare outwardly towards the uppermost side of said handle portion; and b. the uppermost side of said handle portion forms an arch-like extension facing away from the test chamber, and spanning between said lateral sides. a body of non-magnetic inert material adapted to be slidably engaged in a receptacle of an apparatus for performing optical analysis of a fluid sample, said body having a domed cross section transverse to a longitudinal axis with an optically clear flat land portion adapted to be positioned in a field of observation for optical analysis; a test chamber underlying said flat land portion adapted to be filled vertically along the longitudinal axis with said fluid sample; and a vestibule having an enlarged mouth at an entrance end and communicating with the test chamber through an inlet opening opposite the entrance end, said vestibule being disposed above said test chamber when the cartridge is vertical and adapted to act as an overflow reservoir capable of being sealed both from said test chamber and an exterior environment, when a stopper is provided for sealing the vestibule at both the entrance end and the inlet opening, said body further comprising a handle portion comprising two lateral sides with grips (37, 38) and an uppermost side with an opening for receiving the stopper, said handle portion having an inner framed structure that is bounded laterally by two keeper elements (77, 77) for keeping the stopper in place and along the longitudinal axis by said uppermost portion and by the enlarged mouth of said vestibule, said cartridge being characterised in that: a. the lateral sides of the handle portion flare outwardly towards the uppermost side of said handle portion; and b. the uppermost side of said handle portion forms an arch-like extension facing away from the test chamber, and spanning between said lateral sides. a. the lateral sides of the handle portion flare outwardly towards the uppermost side of said handle portion; and b. the uppermost side of said handle portion forms an arch-like extension facing away from the test chamber, and spanning between said lateral sides.