neous sensations, having little difference of local colour, is
an illustration of this principle. (See the _Revue Philosophique_,
September, 1880.)

[21] Even the fusion of elementary sensations of colour, on the
hypothesis of Young and Helmholtz, in a seemingly simple sensation may
be explained to some extent by these circumstances, more especially the
identity of local interpretation.

[22] The perception of lustre as a single quality seems to illustrate a
like error. There is good reason to suppose that this impression arises
through, a difference of brightness in the two retinal images due to the
regularly reflected light. And so when this inequality of retinal
impression is imitated, as it may easily be by combining a black and a
white surface in a stereoscope, we imagine that we are looking at one
lustrous surface. (See Helmholtz, _Physiologische Optik_, p. 782, etc.,
and _Populaere wissenschaftliche Vortraege_, 2tes Heft, p. 80.)

[23] The conditions of the production of these double images have been
accurately determined by Helmholtz, who shows that the coalescence of
impressions takes place whenever the object is so situated in the field
of vision as to make it practically necessary that it should be
recognized as one.

[24] These illusions are, of course, due in part to inattention, since
close critical scrutiny is often sufficient to dispel them. They are
also largely promoted by a preconception that the event is going to
happen in a particular way. But of this more further on. I may add that
the late Professor Clifford has argued ingeniously against the idea of
the world being a continuum, by extending this idea of the wheel of
life. (See _Lectures and Essays_, i. p. 112, _et seq._)

[25] It is supposed that in the case of every sense-organ there is
always some minimum forces of stimulus at work, the effect of which on
our consciousness is _nil_.

[26] See Helmholtz, _Physiologische Optik_, p. 603. Helmholtz's
explanation is criticised by Dr. Hoppe, in the work already referred to
(sec. vii), though I cannot see that his own theory of these movements
is essentially different. The apparent movement of objects in vertigo,
or giddiness, is probably due to the loss, through a physical cause, of
the impressions made by the pressure of the fluid contents of the ear on
the auditory fibres, by which the sense of equilibrium and of rotation
is usually received. (See Ferrier, _Functions of the Brain_, pp. 60,
6