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.

For example, we tend to magnify the differences of light among objects, all of which are feebly illuminated, that is to say, to see them much more removed from one another in point of brightness than when they are more strongly illuminated.