ere are many others which are imprinted
without our knowledge. We fail to notice them because our attention is
directed to something else. But at a subsequent period, when the mind is
in a passive state, these impressions may suddenly revive owing to the
phenomenon of recurrence. This observation may afford an explanation of
some of the phenomena connected with ocular phantoms and hallucinations
not traceable to any disease. In these cases the psychical effects
produced appear to have no objective cause. Bearing in mind the numerous
visual impressions which are being unconsciously made on the retina, it
is not at all unlikely that many of these visual phantoms may be due to
objective causes.

FOOTNOTES:

[19] As an instance of this I may mention the experiment which I saw on
the quick fusion of metals exhibited at the Royal Institution by Sir
William Roberts-Austen (1901), where, owing to the glare and the dense
fumes, it was impossible to see what happened in the crucible. But I was
able to see every detail _on closing the eyes_. The effects of the
smoke, being of less luminescence, cleared away first, and left the
after-image of the molten metal growing clearer on the retina.

[20] E. W. Scripture, _The New Psychology_, p. 101.

CHAPTER XX

GENERAL SURVEY AND CONCLUSION

We have seen that stimulus produces a certain excitatory change in
living substances, and that the excitation produced sometimes expresses
itself in a visible change of form, as seen in muscle; that in many
other cases, however--as in nerve or retina--there is no visible
alteration, but the disturbance produced by the stimulus exhibits itself
in certain electrical changes, and that whereas the mechanical mode of
response is limited in its application, this electrical form is
universal.

This irritability of the tissue, as shown in its capacity for response,
electrical or mechanical, was found to depend on its physiological
activity. Under certain conditions it could be converted from the
responsive to an irresponsive state, either temporarily as by
anaesthetics, or permanently as by poisons. When thus made permanently
irresponsive by any means, the tissue was said to have been killed. We
have seen further that from this observed fact--that a tissue when
killed passes out of the state of responsiveness into that of
irresponsiveness; and from a confusion of 'dead' things with inanimate
matter, it has been tacitly assumed that inorganic