Mr Stevenson’s Experiments on Lights in Rapid Motion. 275 
In experimenting on this subject, I used the apparatus for- 
merly employed by Captain Hall. It consisted of an octa- 
gonal frame, which carried eight of the discs that compose 
the central part of Fresnel’s compound lens, and was suscep- 
tible of being revolved slowly or quickly at pleasure, by means 
of a crank handle and some intermediate gearing. The experi- 
ments were nearly identical with those made by Captain Hall, 
who contrasted the effect of a single lens at rest, or moving 
very slowly, with that produced by the eight lenses, revolv- 
ing with such velocity as to cause an apparently continuous 
impression on the eye. To this experiment I added that 
of comparing the beam thrown out by the central portion 
of a cylindric refractor, such as is used at the fixed light of 
the Isle of May, with the continuous impression obtained by 
the rapid revolution of the lenses. Captain Hall made all his 
comparisons at the short distance of 100 yards; and in order 
to obtain some measure of the intensity, he viewed the lights 
through plates of coloured glass until the luminous discs be- 
came invisible to the eye. I repeated these experiments at 
Gullan, under similar circumstances, but with very different 
results. I shall not, however, enter upon the discussion of 
these differences at present, although they are susceptible of 
explanation, and are corroborative of the conclusions at which 
I have arrived, by comparing the lights from a distance of 14 
miles ; but shall proceed to detail the more important results 
which were obtained by the distant view. Several members 
of the Royal Society witnessed the results of the experiments 
which I shall briefly describe in the following order :— 
1. The flash of the lens revolving slowly was very much 
larger than that of the rapidly revolving series ; and this de- 
crease of size in the luminous object presented to the eye, 
became more marked as the rate of revolution was accele- 
rated, so that, at the velocity of 8 or 10 flashes in a second, 
the naked eye could hardly detect it, and only a few of the 
observers saw it; while the steady light from the refractor 
was distinctly visible. 
2. There was also a marked falling off in the brilliancy of 
the rapid flashes as compared with that of the slow ones; but 
