By SYLVANUS P. THOMPSON, B.A., D.Sc., F.RA.S., 
(Professor of Experimental Physics, University College, Bristol.) 
TN the Transactions of various learned bodies — the Royal 
■^Society, the British Association, &c. — papers have appeared 
from time to time describing various Optical Illusions. Some of 
these illusions have depended upon the duration of retinal im- 
pressions, some upon the formation of accidental subjective 
images, some upon the dispersion or irradiation of the eye, and 
some upon the phenomena of binocular vision. The illusions to 
be described in the present paper do not fall exclusively under 
any one of the heads enumerated, though they depend upon the 
duration of visual impressions, and upon a further and less per- 
fectly understood property of the retina. They are all dependent 
upon motion, either of the object or of the observer, or of both. 
In each case that will be here brought forward there is a movement 
of the object across the field of view, and consequently of the 
image across some portion of the retina. 
The most frequent illusions which arise thus are those in which 
one form of motion apparently takes some other form. As a most 
familiar instance of this kind of illusion we may take the case of 
the apparent motion of trees, hedgerows, and houses, as seen from 
a rapidly-running railway-train, the deception of the senses being 
most complete when the personal sense of motion is least. 
1 The greater part of this article was read before the British Association, at 
Plymouth, in September, 1877, an abstract only having as yet been published. 
A few additional facts were recently communicated to the Bristol Naturalists’ 
Society, and are embodied herewith. — S.P.T. 
Vol. III. B 
