With respect, however, to the first sub-class of these illusions, namely, those arising from organic peculiarities which give a twist, so to speak, to the sensation, no very marked contrast between the different senses presents itself.  So that in illustrating this group we shall be pretty equally concerned with the various modes of perception connected with the different senses.

It may be said once for all that in thus marking off from one another certain groups of illusion, I am not unmindful of the fact that these divisions answer to no very sharp natural distinctions.  In fact, it will be found that one class gradually passes into the other, and that the different characteristics here separated often combine in a most perplexing way.  All that is claimed for this classification is that it is a convenient mode of mapping out the subject.

CHAPTER IV.

ILLUSIONS OF PERCEPTION—­continued.

A. Passive Illusions (a) as determined by the Organism.

In dealing with the illusions which are related to certain peculiarities in the nervous organism and the laws of sensibility, I shall commence with those which are connected with certain limits of sensibility.

Limits of Sensibility.

To begin with, it is known that the sensation does not always answer to the external stimulus in its degree or intensity.  Thus, a certain amount of stimulation is necessary before any sensation arises.  And this will, of course, be greater when there is little or no attention directed to the impression, that is to say, no co-operating central reaction.  Thus it happens that slight stimuli go overlooked, and here illusion may have its starting-point.  The most familiar example of such slight errors is that of movement.  When we are looking at objects, our ocular muscles are apt to execute very slight movements which escape our notice.  Hence we tend, under certain circumstances, to carry over the retinal result of the movement, that is to say, the impression produced by a shifting of the parts of the retinal image to new nervous elements, to the object itself, and so to transform a “subjective” into an “objective” movement.  In a very interesting work on apparent or illusory movements, Professor Hoppe has fully investigated the facts of such slight movements, and endeavoured to specify their causes.[17]

Again, even when the stimulus is sufficient to produce a conscious impression, the degree of the feeling may not represent the degree of the stimulus.  To take a very inconspicuous case, it is found by Fechner that a given increase of force in the stimulus produces a less amount of difference in the resulting sensations when the original stimulus is a powerful one than when it is a feeble one.  It follows from this, that differences in the degree of our sensations do not exactly correspond to objective differences.