A travelling microscope is focused on a small mark on a table. A glass slab $3.0\ \text{cm}$ thick (refractive index $1.5$) is laid over the mark, and a layer of water $4.0\ \text{cm}$ deep (refractive index $4/3$) is poured on top of the slab, as shown in the figure. To see the mark sharply again, the microscope must be

Fig. 1: A microscope above a mark on a table. On the mark lies a glass slab 3.0 cm thick of refractive index 1.5, and on the slab a layer of water 4.0 cm deep of refractive index 4/3.