If the speed of light in carbon disulfide and vacuum is X and Y...

The speed of light in a vacuum is the maximum speed at which light can travel.

When light enters any material medium, its speed decreases due to the optical density of the medium.

The refractive index of a medium is given by $\mu = \dfrac{Y}{X}$, where $Y$ is the speed of light in vacuum and $X$ is the speed of light in the medium.

Since the refractive index of carbon disulfide is greater than $1$ (\mu > 1$), we have \dfrac{Y}{X} > 1$.

This implies $X < Y$.

Answer: $X < Y$