Four students $\mathrm{P},\mathrm{Q},\mathrm{R}$ and $\mathrm{S}$ traced the path of a ray of light passing through a glass slab for an angle of incidence ${40}^{\circ }$ and measured the angle of refraction. The values as measured them were ${18}^{\circ };{22}^{\circ };{25}^{\circ }$ and ${30}^{\circ }$ respectively. The student who has performed the experiment methodically is