348 Scientific Proceedings, Royal Dublin Society. 
The system of revolving lights is, however, subject to this dis- 
advantage, that during the periods of darkness which succeed each 
exhibition of the light the position of the light is lost, and in some 
states of the weather, especially in thick weather, is not again 
easily found. It is no wonder that sailors should consider it a 
consummation devoutly to be wished that the superior power and 
characteristic appearance of the revolving light should be com- 
bined with the permanent continuity of the fixed light. 
About the year 1820, Capt. Basil Hall, R.N., conceived the 
idea that if annular lenses were caused to revolve with such 
velocity that the beam from each should blend into a continuous 
light (like the luminous circle caused by whirling through the air 
any incandescent object), this desideratum would be attained. The 
matter was carefully investigated by the Commissioners of Northern 
Lights, and the sum total of their report was, that the difference 
of volume between the light of a cylindric refractor and that 
produced by the lenses at their greatest velocity was very striking. 
The former presented a large diffuse object of inferior brilliancy, 
while the latter exhibited only a sharp pin point of brilliant light. 
They were therefore discouraged from attempting to improve 
the visibility of fixed lights in the manner proposed by Captain 
Hall. 
Thus matters remained until the subject was again brought to 
the front by the French lighthouse authorities. Certain French 
physiologists had pronounced the opinion that the power of any 
light can be enjoyed to the full if it be presented to the eye for 
one-tenth of a second. Acting on this physiological law, the 
engineers of the French lighthouse department availed themselves 
of it, and constructed the new Cape La Heve lights. These 
lights only remain in the eye one-tenth of a second, and recur 
every five seconds. 
The new light to which I now desire to call attention rests 
in the eye one-fourth of a second, and, unlike the French light, 
never leaves it, z.e. the light is continuously visible. The flashes 
of which it is composed are practically the same in volume and 
intensity as those of the ordinary slowly revolving light, and being 
always in the eye, there is no period of darkness during which 
the sailor may lose the position of the light. Anyone who has 
been: in the habit of looking at revolving lights must have 
