Page:Proceedings of the Royal Society of London Vol 69.djvu/58

50 along the scale and indicated the position. The D line was at 50 on this scale. The temporary scale was afterwards replaced by a permanent one on which the D line was at 17 '7, and to avoid confusion, the early scale readings have been reduced to this scale by subtracting 32'3 from each.

For this, the slits were removed and the spectrum was received on a strip of celluloid placed between a glass plate and a glass Zeiss milli- metre scale. Various substances were then placed on the arc, such as Li, K, Na, Mg, and the bright lines read. A H-tube was also placed at the collimator slit. The results are plotted in Curve No. 1 :

No. 1. Curve connecting the scale readings with the corresponding ware-lengths.