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Notice of Experiments regarding the Visibility of Lights in ra- 
pid Motion, made with a view to the Improvement of Light- 
houses, and of some peculiarities in the impressions made by 
them on the Eye. By Auan Srzvenson, LL.B., F.R.S.E., 
Civil Engineer. Communicated by the Author. 
In the spring of 1836, my attention was called, by our dis- 
tinguished countryman Captain Basil Hall, to a plan by which 
he proposed to increase the intensity and power of fixed lights 
for lighthouses, to such an extent as to render their constant — 
effect little inferior to that of the bright flashes which alter- 
nate with the dark intervals in revolving lights. Since that 
time Captain Hall has made some experiments on the subject, 
which he has described in the United Service Journal ; and I 
have lately, at his request, and with the sanction of the North- 
ern Lights Board, repeated his experiments, and also tried 
some others which appeared to me to bear on the subject. 
The object of this notice is to state the results which were 
obtained from these experiments, as well in regard to the ob- 
ject for which they were originally designed, as in reference 
to some curious phenomena connected with the distribution of 
light and its effect in producing impressions on the eye, which 
were observed in the course of these trials. It is necessary, 
however, that I should, in the first place, give a short account 
of the instruments which were employed and describe their 
action, so that the purpose for which the experiments were 
made may be brought fully into view, and the nature of the 
results which they afforded may be made more intelligible to 
those who have not previously given any consideration to the 
subject. 
In revolving lights on the dioptric principle, the annular 
lens of Fresnel is employed. This instrument consists of a 
centre lens in one piece, and several concentric zones arranged 
so as to form a square of about 900 inches of surface; and it 
possesses the property of projecting to the horizon, in the form 
of one pencil or beam, all the light which falls on its inner face 
from a lamp placed in its principal focus. The consequence 
