A negative placed in an enlarger in the conventional way, emulsion side down and held along its edges, will "pop" when the heat from the lamphouse hits it. But it will pop upwards. In effect the negative flattens itself against the top glass. By omitting the bottom glass two dust collecting surfaces are eliminated. In my darkroom the top glass is never turned over once the underside, the side touching the negative, is dead clean. Dust does not fall upwards. The top surface of the glass is checked in the enlarger beam and any specks are brushed off immediately before putting the carrier in the enlarger.