IN his paper on the manganese spectrum (Phil. Trans. Roy. Soc., vol. 223 A, p. 127) Catalán gives some triplets and multiplets of the spectrum Mn II, and suggests identifications of some of them. I have recently made an attempt to estimate terms from some of these lines; two independent methods agree within the estimated limits of error, and it seems that it is possible thereby to estimate the ionisation potential with an error probably less than half a volt.