The chemical composition of ultra-high-energy cosmic rays (UHECRs) affects the observable distribution of air-shower $X_{\rm max}$ values, the atmospheric slant depth at which the number of secondary shower particles reaches its maximum. The observed $X_{\rm max}$ distributions at various primary UHECR energies can be compared with the distributions predicted by detailed detector simulations for any assumed composition and high-energy hadronic interaction model. In this poster, we present measurements of $X_{\rm max}$ by the Telescope Array (TA) fluorescence detectors with stereoscopic shower reconstruction. We find that for all hadronic models considered, the data collected since TA operation began in 2007 is consistent with a chiefly light UHECR composition.