A travelling microscope is used to determine the refractive index of a glass slab. If \( 40 \) divisions are there in \( 1 \, \text{cm} \) on main scale and \( 50 \) Vernier scale divisions are equal to \( 49 \) main scale divisions, then least count of the travelling microscope is _______ \( \times 10^{-6} \, \text{m} \).
