of Edinburgh, Session 1878-79. 
41 
river. Sir D. Brewster made some experiments while looking out 
of rapid-moving trains on the effect of images moving rapidly across 
the retina, but he does not seem to have observed these motion 
spectra. Professor Silvanus P. Thompson has, however, observed 
a somewhat similar phenomenon.* He says : — “ If, from a rapid 
railway train, objects from which the train is receding be watched, 
they seem to shrink as they are left behind, their images contracting 
and moving from the edges of the retina towards its centre. If, 
after watching this motion for some time, the gaze be transferred to 
an object at a constant distance from the eye, it seems to be actually 
expanding and approaching.” 
Under ordinary circumstances motion .spectra are not easily 
noticed. Like form and colour spectra they require special precau- 
tions to be taken before they can be distinctly observed. I find, 
however, that these precautions are extremely simple, and that but 
little apparatus is required for showing them distinctly. 
Simply take a circular cardboard disc,f painted as shown in 
fig. 1, and mount it on a hori- 
zontal shaft. How, if, while 
this disc is being steadily ro- 
tated,:}; the eye is fixed upon 
jt, and the motion continued a 
short time and then stopped, 
the disc will at once appear to 
begin rotating in the opposite 
direction. The experiment is, 
however, improved by painting 
either a copy of the disc, or a 
wheel, or any other object, on 
a sheet of paper, and hanging 
it up near the rotating disc. If, after looking at the rotating disc 
* Report of the British Association, 1877. 
t The discs used in the experiments were from 22 cm. to 44 cm. in diameter. 
The number of parts into which the disc is divided does not seem to be of 
much importance. If divided into fewer parts, then it simply requires to be 
driven at a greater velocity. If divided into 24 parts, it can easily be driven 
quick enough with a handle on the opposite end of the shaft without the aid 
of multiplying gear. 
J The disc should be rotated at such a rate that the eye cannot distinguish the 
black and white divisions, but not so quick that they are blended together. 
