OF ILLUMINATING LIGHTHOUSES. 
223 
limits to make reflectors four feet in diameter, and seven inches focal distance, we 
might by employing three such reflectors with lamps 2 \ inches in diameter, having 
two concentric wicks, produce the same illuminating power and the same durations 
as the Beechy Head lighthouse, the vertical divergence being 14°, and the consump- 
tion equal to about seventeen or eighteen Argand burners. 
Still, however, there would be only 14° 14' of the horizon illuminated with the 
brightest light, but an improvement may be made in this respect by altering the 
form of the reflector, for as all reflectors of the same diameter have the same illumi- 
nating power when illuminated with the same lamp, that which has the greatest angle 
of minimum divergence, or the longest duration of the bright period, will be that in 
which the distance A F is the shortest, that is to say when 
the focus is in the plane of the end of the reflector, and it 
is not improbable that having by this means less reflect- 
ing surface there would be less absorption and a better 
light. />But in increasing the duration of the brightest 
period we diminish the angles of maximum divergence 
both horizontally and vertically. The divergences of a 
reflector four feet diameter and 7 inches focal distance 
with a lamp 2 \ inches diameter would be. 
Vert. Div. 
15° 22' 
Max. Hor. Div. 
18° 34' 
Min. Hor. Dii 
4° 42' 
whereas if the focus were in the plane of its end they would be 
Vert. Div. Max. Hor. Div. Min. Hor. Div. 
8° 22' 4 10° 46' 5° 22' 
But notwithstanding the great reduction in the angles of maximum divergence, 
I have no hesitation in considering this to be the best form of reflector when the 
vertical divergence required by the situation of the lighthouse will admit of its ap- 
plication, as it gives the longest duration to the bright period ; for if the distance and 
state of the weather be such that it becomes a question whether the light be seen or 
not, the spread of weaker light will of course be invisible, and if the light be seen 
distinctly this weak light is not required, the regular appearance and disappearance 
of the light at known intervals being all that is necessary ; such light, however, ought 
not to be used for purposes of distinction, as its duration is subject to great variations 
from distance or the state of the weather. 
What number of degrees in the horizon it is necessary to illuminate, and more par- 
ticularly what number it is necessary to illuminate with the brightest light, is a ques- 
tion upon which no determination appears to have been arrived at, but there can be 
no doubt in applying a given illuminating power (say that of ten reflectors 21 inches 
diameter,) to a lighthouse, that having attained a sufficient vertical divergence for 
its situation, the rest of the light cannot be better employed than in increasing as 
