portions 38 of the chambers 40 42 have in the central regions thereof respectively upwardly projecting cones 48 the main body 10 has a laterally extending lip 50 around the upper extremity thereof to which the cuvette cover 14 is secured the cover 14 fig2 is preferably a probe puncturable laminate which provides a moisture and gas barrier the construction and use of which is not part of the present invention the chamber 46 may be utilized to hold a liquid sample for example prior to deposit of a portion thereof in one of the chambers 40 42 as shown in fig2 for example the lower end portions of the legs of the groove 32 of generally inverted u shape are gently flared outwardly or inclined as at 33 this permits registration of the leg portions with the respective orifices 30 while enabling narrowing of the upper portion of the inverted u shaped groove 32 so that it may be laterally separated from the optical window 24 as shown in fig2 the aforementioned gently flared portions 33 of the groove 32 minimizes restriction of flow in the passageway 32a further as shown in the last mentioned view the top portion of the groove 32 is shaped on a substantial radius so as to minimize restriction of flow in the passageway the main body 10 and the rear cover portion 12 are so shaped fig1 and 2 that the two may be easily assembled for this purpose the rear cover portion 12 is placed in parallel face to face relation with the recess 26.