WITH regard to Mr. Evans's communication to NATURE, September 4, p. 5, I should like to remark that while I have for some time recognised that the experimental evidence, on the whole, seems to be in favour of helium as the origin of the new lines 4686, c these bands are of very feeble intensity at the low pressures necessary for their approximate isolation in the spectrum of the pure gas, but I have seen them greatly intensified when carbonic oxide was present as an impurity in helium. Also, the Ritz series of infra-red hydrogen lines was found by Paschen to be brighter in a mixture of hydrogen and helium than in hydrogen alone. Apart from this, I find it difficult to believe that the close agreement of one set of lines with the principal series calculated for hydrogen by Rydberg is merely accidental.