Context :

Aims : We study spectral lines in exceptionally bright solar limb prominences with pronounced sodium and magnesium emission and central reversion of the first two hydrogen Balmer lines .

Methods : We simultaneously measure the line profiles of H \alpha , H \beta , HeD _ { 3 } , He II 4685 , He I 5015 ( singlet ) , NaD _ { 2 } and Mgb _ { 2 } using the THEMIS telescope on Tenerife .

Results : We find that most prominences with significant NaD _ { 2 } and Mgb _ { 2 } emission show centrally reversed profiles of H \alpha and occasionally even of H \beta .
The strongest emissions reach integrated intensities E \beta > 16 \cdot 10 ^ { 4 } erg/ ( s cm ^ { 2 } ster ) .
The centrally reversed profiles are well reproduced by semi-infinite models .
The source function reaches S _ { \alpha } \leq 36 \cdot 10 ^ { 4 } erg/ ( s cm ^ { 2 } ster Å ) corresponding to an excitation temperature of T _ { ex } \approx 3950 K ; here , the optical thickness of H _ { \alpha } amounts to \tau \approx 10.0 .
The narrow widths of the NaD _ { 2 } and Mgb _ { 2 } profiles yield a non-thermal broadening of V _ { tu } = 5 km/s .

Conclusions :