Another approach to improving the stability would be to use a purge gas having a smaller refractive index, and consequently, a smaller change in refractive index with temperature. Helium has approximately 10x smaller non-vacuum contribution to the refractive index than air, and a correspondingly smaller dn/dt. This slide does not show what was intended, namely a 10x improvement in stability. However, it does show that purging the spectrometer is itself a serious noise source. The left side of the trace was recorded while the helium flow was 40 cubic feet per hour (the recommended purge rate). As soon as the flow was turned off, the noise was greatly reduced to about 0.06% p-p (compare to 0.11% for "still" air). A useful experiment would have been to purge with air or nitrogen to compare to the disturbance created by helium. It should be 10x worse.