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COLLECTED PHYSICAL PAPERS
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three elements are in good order, no light-message can be perceived. We must have a sensitive retina, and the conducting nerve, free from injury. Finally, just as the galvanometer will fail to detect a current if it is injured by rough usage, so also after a violent blow, the brain will no longer perceive, though the terminal organ, the retina, and the connecting optic nerve may be intact.

Fig. 63. Magnetic Lever Recorder. M muscle; A uninjured, B injured ends. E Eʹ non-polarising electrodes connecting A and B with galvanometer G. Stimulus produces "negative variation" of current of rest. Index connected with galvanometer needle records curve on travelling paper (in practice, moving galvanometer spot of light traces curve on photographic plate). Rising part of curve shows effect of stimulus; descending part, recovery.


Electric Response

If we take a piece of living muscle whose surface is uninjured, then any two points A and B on such a

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