274 Mr Stevenson’s Experiments on Lights in Rapid Motion. 
pect that this should occur in connection with the revolving 
system, because the velocity is an extraneous circumstance, 
by no means necessary to an equal distribution of the light, 
which can, as we already know, be more naturally, and at the 
same time perfectly, attained by the use of the zones. 
On the other hand, it must not be forgotten, that although 
the effect of both methods is to give each part of the horizon 
an equal share of light, there is yet this difference between 
them, that while the light from the zones is equally intense 
at every instant of time, that evolved by the rapidly circulat- 
ing lenses is constantly passing through every phase between 
total darkness and the brightest flash of the lens; and this 
difference, taken in connection with some curious physiological 
observations regarding the sensibility of the retina, gives con- 
siderable countenance to the expectation on which Captain 
Hall’s ingenious expedient is based. The fact which has al- 
ready been noticed, and which the beautiful experiments of 
M. Plateau and Professor Wheatstone have of late rendered 
more precise, that the duration of an impression on the retina 
is not only appreciable, but is much greater than the time re- 
quired to cause it, seems to encourage us in expecting, that 
while the velocity required to produce continuity of effect 
would not be found so great as to interfere with the formation 
of a full impression, the duration of the impulse from each 
flash would remain unaltered, and the dark intervals which 
do not excite the retina would, at the same time, be shorten- 
ed, and that, therefore, we might in this manner obtain an 
effect exceeding the brilliancy of a steady light distributed 
equally in every direction by the ordinary method. Many 
persons, indeed, who have speculated on this subject, seem to 
be of opinion, that, so far from the whole effect of the series of 
continuous impressions being weakened by a blending of the 
dark with the bright intervals, the eye wouldin reality be stimu- 
lated by the contrast of light and darkness, so as thereby to re- 
ceive amore complete and durable impulse from the light. It 
is obvious, however, that this question regarding the probable 
effect to be anticipated by a revolution so rapid, as to cause a 
continuous impression, can only be satisfactorily answered by 
an appeal to experiment. 
