224 
MR. W. H. BARLOW ON DIFFERENT MODES 
much as possible the length of the period during which the full power of the instru- 
ment is acting, or the number of degrees in the horizon illuminated with the full 
power of the instrument, however much the spread of weaker light may be dimi- 
nished by so doing. 
From the foregoing it would, appear that the best mode of applying lenses and re- 
flectors to lighthouses when illuminated with oil is as follows. 
When less than 3° of vertical divergence is required. 
The French lens is to be preferred with its additional apparatus by which 32° 82 f 
of the horizon is illuminated with a light equal to 10‘4 reflectors 21 inches diameter, 
the sum of the angles of maximum horizontal divergence being 42° O', and the con- 
sumption of oil equal to fifteen or sixteen Argand burners. 
For more than 3° and less than 8° vertical divergence. 
Reflectors four feet diameter should be employed, if practicable, with the focus in 
the plane of the end. Four such reflectors with lamps 2- inches diameter having 
two concentric wicks would illuminate 21° 28' of the horizon, with a light equal in 
power to 10*4 reflectors 21 inches diameter, the sum of the angles of maximum hori- 
zontal divergence being 43° 4', and the probable consumption equal to twenty-three 
Argand burners. 
Or three such reflectors with lamps 2\ inches diameter having three concentric 
wicks would illuminate 17 ° 54' of the horizon, with a light equal in power to fifteen 
reflectors 21 inches diameter, the sum of the angles of maximum horizontal diver- 
gence being 35° 54', and the probable consumption equal to thirty Argand burners. 
For more than 8° and less than 15° vertical divergence. 
Reflectors four feet in diameter may be employed with the focal distance so ar- 
ranged as to give the vertical divergence required. 
And for more than 15° vertical divergence it will perhaps be found better to use 
smaller reflectors and more in number. This last, however, is a case which will 
rarely occur in lights where great illuminating power is required. 
It is proper, however, to observe generally, that the lens is liable to one practical 
objection, viz. that depending on one light only, any accident whereby it becomes ex- 
tinguished leads to total darkness, which is not so likely to happen in a system of 
reflectors which has several lights to depend on. 
