17.5 Given that the area of $\triangle A B C$ is $S$, points $D$, $E$, and $F$ are the midpoints of $B C$, $C A$, and $A B$ respectively, $I_{1}$, $I_{2}$, and $I_{3}$ are the incenters of $\triangle A E F$, $\triangle B F D$, and $\triangle C D E$ respectively, and the area of $\triangle I_{1} I_{2} I_{3}$ is $S^{\prime}$. Find the value of $S^{\prime} / S$.