Mr Stevenson’s Experiments on Lights in Rapid Motion. 277 
tant lights depends on the volume of the impression, in a 
greater degree than has perhaps been generally imagined. 
5. That as the size and intensity of the radiants causing 
these various impressions to a distant observer, are the same, 
the volume of the light, and, consequently, cwteris paribus, its 
visibility, is, within certain limits, proportionate to the time 
during which the object is present to the eye. 
Such appear to be the general conclusions which these ex- 
periments warrant us in drawing ; and the practical result, in 
so far as lighthouses are concerned, seems sufficient to discou- 
rage us from attempting to improve the visibility of fixed lights 
in the manner proposed by Captain Hall, even supposing the 
practical ditficulties connected with the great centrifugal force 
generated by the rapid revolution of the lenses to be less than 
they really are. 
I shall be excused, I hope, for saying a few words in con- 
clusion regarding the decrease in the volume of the luminous 
object caused by the rapid motion of the lights. This effect 
is interesting, from its apparent connection with the curious 
phenomenon of irradiation. When luminous bodies, such as 
the lights of distant lamps, are seen by night, they appear 
much larger than they would do by day; and this effect is 
said to be produced by irradiation. M. Plateau, in his elabo- 
rate essay on this subject, after a careful examination of all 
the theories of irradiation, states it to be his opinion, that the 
most probable mode of accounting for the various observed 
phenomena of irradiation is to suppose, that, in the case of a 
night-view, the excitement caused by light is propagated over 
the retina beyond the limits of the day-image of the object, 
owing to the increased stimulus produced by the contrast of 
light and darkness ; and he also lays it down as a law confirmed 
by numerous experiments, that irradiation increases with the 
duration of the observation. It appears, therefore, not un- 
reasonable to conjecture, that the deficiency of volume ob- 
served during the rapid revolution of the lenses may have been 
caused by the light being present to the eye so short a time, 
that the retina was not stimulated in a degree sufficient to 
produce the amount of irradiation required for causing a large 
visual object. When, indeed, the statement of M. Plateau, 
