Mr Stevenson’s Experiments on Lights in Rapid Motion. 271 
of this action is, that when several lenses are so arranged as 
to form a right prism which circulates round a lamp placed in 
the common focus, a distant observer receives from each lens, 
as its axis crosses his line of vision, a bright flash, which is 
succeeded by total darkness, when one of the dark spaces in- 
termediate between the beams passes over his eye; and this 
succession of bright flashes alternating with dark intervals, 
produces the characteristic appearance of a revolving light. 
The fixed light, on the other hand, presents to the eye a 
steady and unchanging appearance; and the chief object to 
be obtained in its construction, is to unite the greatest bril- 
liancy with an equal distribution of the light in every direc- 
tion. This condition of perfect distribution is most rigorously 
fulfilled by the use of refracting zones or belts, which form, 
by their union, a cylinder enveloping the flames placed in its 
centre. This cylinder is a true solid of revolution, generated 
by the rotation of the mixtilinear central section of a great 
annular lens, round a vertical axis passing through its princi- 
pal focus ; and must, therefore, possess the property of refract- 
ing the light in the vertical direction only, without affecting 
its natural divergence horizontally. The light which is inci- 
dent from the focus on the inner surface of the belt is there- 
fore projected forwards in the shape of a flat ring of equal 
brilliancy all round the horizon. 
This very brief account of the instruments used in the fixed 
and revolving lights on the dioptric principle will, it is hoped, 
be found sufficient to render intelligible the following outline 
of the plan proposed by Captain Hall for the improvement of 
fixed lights, and the account of the trials that were made with 
this object in view. 
The familiar experiment of whirling a burning stick rapidly 
round the head, so as to produce a ribbon of light, proves the 
possibility of causing a continuous impression on the retina 
by intermittent images succeeding each other with a certain 
velocity. From the moderate velocity at which this conti- 
nuity of impression is obtained, we should be warranted in 
concluding, a priori, that the time required to make an im- 
pression on the retina is considerably less than the duration 
