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Proceedings of the Boyal Society 
rays of light strike the retina, and the production of the sensation ; 
and further, the sensation remains for some time after withdrawal 
of the stimulus. It is unlikely, therefore, that the energy of light 
is directly transformed into nerve energy. 
In shortly summing up the chief points alluded to in this 
paper, it may he stated — 
(1) That stimuli applied to sensitive peripheral end-organs with 
increasing frequency, produce at first consciousness of the separate 
stimuli ; then, their individual characters are lost, and a disagreeable 
sensation — “ roughness” in the case of tactile feeling — is produced; 
and lastly, we feel a sensation indistinguishable from a constantly 
applied stimulus. 
(2) For the development of nerve energy from external stimuli 
in a sensory nerve a latent period is necessary, and it elapses be- 
tween the application of the stimulus and the resultant effect on 
the nerve. On withdrawing the stimulus, the nerve is still in a 
condition of activity, and remains so for a certain period (after- 
period). When this nervous energy is transformed into nerve-cell 
energy in the sensory centres, there is probably neither a well- 
marked latent nor an after-period. 
(3) When, as in the sense of touch, the consciousness of the 
stimulus corresponds more exactly in time with the application of 
the stimulus than in the sense of sight, we are to look not to any 
differences in the limitation of consciousness itself, but to the time 
elapsing in each case before the nerve is excited by the stimulus 
and the length of the corresponding after-period. 
2. The Old English Mile. By Wm. Flinders Petrie. 
Communicated by Professor Eobertson Smith. 
The length of the old English mile has been hitherto so uncertain, 
that any fresh light upon it is well worthy of study ; and an im- 
portant source of information — the map in the Bodleian Library — 
has not yet been brought to bear upon the question. The present 
inquiry was suggested by the sight of this map, which seems to add 
so much to our knowledge that a review of the whole subject has 
become desirable. It is proposed, therefore, in this paper to bring 
together all the data worth consideration, beginning with the most 
