WE beg to thank Mr. Gordon for drawing attention to the fact that the principle of rotation of plane of polarisation of light in a magnetic field could not actually be employed with the form of receiver symbolically described by us in NATURE, vol. xxi. p. 589. Having satisfied ourselves that there could be no doubt of the feasibility of using the first form of apparatus, which we spoke of, as a receiver in a sight telegraph, we merely wished to point out, at the end of our letter, that other methods might perhaps be employed; and we still have no doubt that with a certain proper arrangement of the apparatus not only the effects observed by Dr. Kerr, but other of the Faraday polarisation of light effects might be practically made use of. For it must be remembered that the actual electric currents now used to transmit articulate speech are only one forty-millionth per cent as strong as those necessary to work even a delicate telegraph relay, whereas it required several Grove's to show in a decided way the old experiment of the sound emitted by an iron bar on being magnetised.