nfluence of the temporary state of activity of adjacent
retinal elements. To me, however, these appear to be illusions, since
they may be brought under the head of wrong _interpretations_ of
sense-impressions. When we see a grey patch as rose-red, as though it
were so independently of the action of the complementary light
previously or simultaneously, that is to say, as though it would appear
rose-red to an eye independently of this action, we surely misinterpret.

[35] Quoted by G.H. Lewes, _loc. cit._, p. 257.

[36] The subject of the perception of movement is too intricate to be
dealt with fully here. I have only touched on it so far as necessary to
illustrate our general principle. For a fuller treatment of the subject,
see the work of Dr. Hoppe, already referred to.

[37] The perception of magnitude is closely connected with that of
distance, and is similarly apt to take an illusory form. I need only
refer to the well-known simple optical contrivances for increasing the
apparent magnitude of objects. I ought, perhaps, to add that I do not
profess to give a complete account of optical illusions here, but only
to select a few prominent varieties, with a view to illustrate general
principles of illusion. For a fuller account of the various mechanical
arrangements for producing optical illusion, I must refer the reader to
the writings of Sir D. Brewster and Helmholtz.

[38] Painters are well aware that the colours at the red end of the
spectrum are apt to appear as advancing, while those of the violet end
are known as retiring. The appearance of relief given by a gilded
pattern on a dark blue as ground, is in part referable to the principle
just referred to. In addition, it appears to involve a difference in the
action of the muscles of accommodation in the successive adaptations of
the eye to the most refrangible and the least refrangible rays. (See
Bruecke, _Die Physiologie der Farben_, sec. 17.)

[39] Helmholtz tells us (_Populaere wissenschaftliche Vortraege_, 3tes
Heft, p. 64) that even in a stereoscopic arrangement the presence of a
wrong cast shadow sufficed to disturb the illusion.

[40] Among the means of giving a vivid sense of depth to a picture,
emphasized by Helmholtz, is diminishing magnitude. It is obvious that
the perceptions of real magnitude and distance are mutually involved.
When, for example, a picture represents a receding series of objects, as
animals, trees, or buildings, the sense of the t