TRANSACTIONS OF SECTION A. 153 



An atom with K,L_,M orbits is known to respond to K,L,M frequencies of 

 X radiation, and to be ionised thereby. 



Outlying electrons, such as are often held responsible for chemical or molecular 

 processes, could respond to lower frequencies characteristic of visible light. 



And if the orbital radius is estimated which shall enable an electron to 

 respond to red, green, or violet light, the order of magnitude is not much 

 larger than the atomic size 10'^ centimetre, even for heavy atoms. The necessary 

 radius varies as the cube root of Moseley's atomic number, and with the two- 

 thirds power of the wave length. 



The suggestion is that the retina may be found to contain atoms in such a 

 condition of incipient instability ('sub-generative,' as Professor Eccles calls 

 the state of certain wireless ' valves ') as to be readily excited by cumulative 

 impulses of the right luminous frequency, and thereby to be stimulated so as 

 to expel an electron and excite a nerve. The energy of expulsion could only 

 be attributable to the incident light on the principle of syntonic accimaulation — 

 the ionisation-energy might be represented as hn — and so the extreme sensi- 

 tiveness of the eye would be accounted for. The atom would be an amplifier 

 or relay, able to respond to the faintest vibration of the right frequency. Re- 

 tinal fatigue and other phenomena of vision could also be accounted for. 



If, however, vision is tri-chromic, the tuning must not be too precise, the 

 responders must be somewhat damped so as to respond over a fair range ; and 

 in the current Phil. Mag. (September 1919) Professor Barton, of Nottingham, 

 claims to have shown by mechanical experiments on damped pendulums that 

 three suitably damped and connected vibrators will exhibit phenomena which 

 by an effort may be regarded as analogous to colour-vision. 



Whether three varieties of vibrator are sufficient, or whether more are 

 necessary, makes no difference to the present communication, which is intended 

 to suggest to physico-physiological experimenters the attempt to examine whether 

 chemical substances can be found in a recently removed retina which are able 

 to emit high-speed electrons when subjected to light. 



THURSDAY, Sb]PT EMBER 11. 



The foMowing Papers and Reports were read : — 



1. The ilanisation of Argon and Helium by Electron Collisions. 

 By Professor F. Horton and Miss A. C. Davies.^ 



Experiments were described showing that there are two critical velocities 

 for eleotroins in both Argon and Helium. At the lower critical velocity, radiation 

 is produced from the gas ; at the higher critical velocity ionisation of the gas 

 takes place. In the case of Argon these velocities correspond to potential 

 differences of 11-5 volts and 15T volts respectively, and in Helium to potential 

 differences of 20-4 volts and 25-6 volts respectively. From the values found for 

 the ionisation velocities the high-frequency limits of the spectra of the two 

 gases were calculated by applying the quantum relation e V = /i n and it was 

 shown that these Imiits agree with those recently determined spectroscopically 

 by Lyman. 



2. The Production of Luminosity in Helium by Electron Coliisions. 

 By Professor F. Horton and Miss J). Bailey. " 



Helium atoms were bombarded by electrons, the veiocity of which was 

 gradually increased until luminosity was produced in the gas. It was found 

 that the electron velocity could then be decreased slightly and the luminosity 

 maintained. Experiments were made to determine the least velocity of the 



1 Partly published in Proc. Roy. Soc. A, vol. 95, p. 408. Remainder to be 

 published in the same. 



- To be published in 7*^(7. Mag. 



