Sensitivity of the selected diagnostic lines to microturbulence, shown as the differential response of the emergent spectrum to changes in ξ. Each row corresponds to a dwarf star of spectral type O9, B0, and B1, respectively. The first seven panels show He I singlet (4387, 4922, 6678 Å) and triplet (4026, 4471, 4713, 5876 Å) lines, followed by the three components of the strong Si III triplet (4553, 4568, 4575 Å).