A method of preparing a slide of a biological specimen, including the steps of (a) providing a slide containing a biological specimen and a cover slip, (b) placing a liquid non-evaporating sealing compound such as mineral oil at spaced locations around an area on the slide and (c) placing the cover slip over the specimen area whereby the specimen and reagent is between the slide and the cover slip and the sealing compound spreads to define a closed boundary around the specimen in the space between the slide and the cover slip. Testing of the biological specimen may then be performed automatically.