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, it should not be too hastily concluded that they are not due to hydrogen. Mr. Evans appears to have succeeded in eliminating the ordinary spectroscopic indications of hydrogen from his helium tubes, but is it not possible that, under the special conditions of the strongly disruptive discharge, with helium also present, residual hydrogen may be represented only by the new lines? This would not be the only known case in which the presence of helium aids the development of the spectrum of another gas with which it is mixed. I have observed this effect in the case of the series of bands of carbonic oxide which are characteristic of the tails of comets; 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.