ates that, owing to this tendency, he has occasionally caught
himself, on a dark night, entertaining the illusion that the
comparatively bright objects visible in twilight were self-luminous.[18]

Again, there are limits to the conscious separation of sensations which
are received together, and this fact gives rise to illusion. In general,
the number of distinguishable sensations answers to the number of
external causes; but this is not always the case, and here we naturally
fall into the error of mistaking the number of the stimuli. Reference
has already been made to this fact in connection with the question
whether consciousness can be mistaken as to the character of a present
feeling.

The case of confusing two impressions when the sensory fibres involved
are very near one another, has already been alluded to. Both in touch
and in sight we always take two or more points for one when they are
only separated by an interval that falls below the limits of local
discrimination. It seems to follow from this that our perception of the
world as a continuum, made up of points perfectly continuous one with
another may, for what we know, be illusory. Supposing the universe to
consist of atoms separated by very fine intervals, then it is
demonstrable that it would appear to our sensibility as a continuum,
just as it does now.[19]

Two or more simultaneous sensations are indistinguishable from one
another, not only when they have nearly the same local origin, but under
other circumstances. The blending of partial sensations of tone in a
_klang_-sensation, and the coalescence in certain cases of the
impressions received by way of the two retinas, are examples of this. It
is not quite certain what determines this fusion of two simultaneous
feelings. It may be said generally that it is favoured by similarity
between the sensations;[20] by a comparative feebleness of one of the
feelings; by the fact of habitual concomitance, the two sensations
occurring rarely, if ever, in isolation; and by the presence of a mental
disposition to view them as answering to one external object. These
considerations help us to explain the coalescence of the retinal
impressions and its limits, the fusion of partial tones, and so on.[21]

It is plain that this fusion of sensations, whatever its exact
conditions may be, gives rise to error or wrong interpretation of the
sense-impression. Thus, to take the points of two legs of a pair of
compasses for o