234 
[March, 
Some New Optical Illusions . 
pounds. I called them simple, and yet in reality I gave 
them so many properties that they became each a focus of 
powers instead of mere atoms, and I am willing to look 
farther back as you do for the beginning of things. 
V. SOME NEW OPTICAL ILLUSIONS.* 
By Silvanus P. Thompson, B.A., D.Sc.. F.R.A.S., 
Professor of Experimental Physics, University College, 
Bristol. 
f LN the Transactions of various learned bodies — the Royal 
L Society, the British Association, &c. — papers have ap- 
peared from time to time describing various Optical 
Illusions. Some of these illusions have depended upon the 
duration of retinal impressions, 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 pre- 
sent 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 perfectly 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. 
When the train in which you are seated is drawn up 
beside another train, and then moves slowly forward, 
* The greater part of this article was read before the British Association, at 
Plymouth, in September, 1877, an abstraft only having as yet been published. 
A few additional fadts were recently communicated to the British Naturalists’ 
Society, and are embodied herewith. — S. P. T. 
