43 
of Edinburgh, Session 1878 - 79 . 
centre or the circumference, according to the direction of the motion 
of the real impression. 
These motion spectra are also seen if the eyes are closed after 
looking at the moving body, the spectrum of the moving paper 
band suggesting a phantom shower of rain in sunshine, the direc- 
tion of the apparent motion being the opposite of that of the real 
impression. 
It might be thought, since the spectrum of the moving band 
seen on the mottled paper seems to be in motion, and as some of 
the mottlings seem to flow past the others, that if we were to draw 
a straight line across the spectral stream, the line ought to appear 
bent, because it might be expected that the part of the line in the 
stream would appear to move forwards. Such, however, is not the 
case. So far as my experiments go, I have never seen the least 
appearance of a bend produced in a straight line ; indeed, the 
straight line does much to stop the apparent motion. Again, in the 
first experiment with the circular disc, if we make the drawing at 
which we afterwards look larger than the exciting disc, — as, for 
instance, by extending the spokes of the wheel to a greater size 
than the rotating disc, — then this extension will entirely destroy all 
appearance of rotation, and the wheel will appear at rest. Do not 
these last experiments suggest that the seat of the illusion is deeper 
than the retina 1 ? I shall not, however, attempt to answer this 
question, as the experiments do not point to any definite conclusion. 
Experiments were also made to determine the effect of influencing 
the whole retina. This was done by looking so closely at the 
moving band that its image covered the whole retina, but no de- 
cided effect was noticed. Experiments were also made with the 
same object by means of a large box-shaped arrangement, the sides 
of which were made of tracing paper having vertical bands of black 
paper 4 cm. broad and fixed 4 cm. apart. The observer being seated 
in a chair, the box was let down over him and put in motion, which 
was continued some time ; the box was then raised, but no appear- 
ance of motion in surrounding objects was observed. There were, 
however, some curious effects produced by the rotation of this 
apparatus. At certain times, while surrounded by the rotating box, 
the observer felt as if rotating in the opposite direction. The most 
certain result, however, was a most disagreeable sickening effect* 
