cate that in sodium vapour we have a medium which approaches more nearly to the ideal absorbing medium, with but a single natural period of vibration, than any substance heretofore investigated.
Of course I am speaking here only with reference to the natural vibration which appears to influence the dispersion. Strictly speaking, there are two natural periods, of course, which influence the velocity of the light in the medium, but when the medium is very dense the con- dition certainly approaches very nearly to that of a single period medium. As I have said before, the fluted absorption bands are without influence on the dispersion, at least their influence is too slight to be detected by the methods that have been employed thus far.
In the first part of the present investigation* I have stated, as one of the principal conclusions with regard to the frog's skin, that the normal electrical response of the excited skin is of outgoing direction.
The chief object of the following observations was to ascertain whether or no similar effects of outgoing direction are manifested by the pad of the cat's foot, this having been, since the first observations of Hermann and of Luchsinger, the chosen object upon which to demonstrate the cutaneous (epithelial and glandular) currents aroused by nerve-stimulation.
I have examined the cutaneous currents, A indirectly aroused by nerve-stimulation, B directly aroused by electrical excitation of the skin itself in the manner described and figured in my previous com- munication (loc. cit., p. 481).
A. INDIRECT EXCITATION. The animals were decapitated, and used in the first instance for the observation of indirect effects and their possible modification in consequence of arrested circulation. In every case, without exception, the electrical effect of excitation of the sciatic nerve upon the pads proved to be an ingoing current, as described by Luchsinger and by Hermann.! The effect gradually declined with lapse of time, and disappeared within 1 hour after decapitation, with- out exhibiting any change of sign or other modification.
Although it was not my purpose to pay particular attention to this point, I may take the opportunity of stating that the experiment, as
- "On Skin Currents. Part I. The Frog's Skin," ' Roy. Soc. Proc.,' June 6,
1901, vol. 68, p. 480.
t Kendall and Luchsinger, " Zur Theorie der Seoretionen," ' Pfliiger's Archiv,' Tol. 13, 1876, p. 212 ; Hermann and Luchsinger, " Ueber die Secretionsstrome der Haut bei der Katze," ' Pfliiger's Archiv,' vol. 17, 1878, p. 310.
