The variation with temperature of the refractive index of fused silica for the helium line 5877.2A has been determined between 18° and −200°C and the results have been applied to the calibration of vitreous silica refraction thermometers at low temperatures. These data have been correlated with measurements at high temperatures. It is found that the refractive index between −200°C and 1000°C can be represented by the equationn=1.45657+0.001553 t/100+0.00219 e−0.33t/100.