220 
MR. W. II. BARLOW ON DIFFERENT MODES 
From the above Table it will be seen, that taking the fall power of a reflector 
in connection with the duration due to its maximum divergence, gives a very 
inadequate idea of the quality of the instrument ; and that from the high ratios 
in which the illuminating power increases and diminishes immediately before and 
after the brightest period, the duration due to the minimum divergence is all we can 
depend on, if the distance and state of the weather be such as to require the full 
power of the instrument ; and as it is under circumstances like these that our beacon 
lights are most called for to give their aid to the benighted mariner, the duration of 
the brightest period becomes one of the most essential qualities to be attended to. 
In comparing the lenses with the reflectors in this respect, we find (by referring to 
the above Table) that when the revolution is made so that the time from the ap- 
pearance to the re-appearance of the light is equal in each, the duration of the 
brightest period is as 14 to 6, that is to say, the number of degrees of the horizon 
illuminated with the brightest light by the eight sides of a system of lenses, is to the 
number illuminated with the brightest light by the three sides of a reflector frame as 
14 is to 6 . Consequently we should require seven sides to our reflector frame to be 
equal in this respect to the lenses ; and as each side must have ten reflectors to be 
equal in power to the lens with its additional apparatus, we should require to con- 
sume the oil of seventy Arganb lamps to produce the same effect with the present 
construction of our reflectors, as that which is obtained in the lenses with the con- 
sumption of fifteen or sixteen Argand lamps. 
Before, however, we give an opinion as to whether the lens or the reflectors is the 
best instrument for our lighthouses, or before we examine whether any improvement 
can be made in them, we must first consider the situation and purposes of the light- 
houses to which they are to be applied. 
On the Situations and Purposes of Lighthouses , and the Application of Lenses and Re- 
flectors to the Duties required in them. 
Lighthouses may be divided into three classes, viz. 
First. Beacon or warning lights, being those which are placed to warn a vessel, 
and to which she can never be nearer than three or four miles. 
Secondly. Guiding or leading lights, being those which are placed to guide a vessel, 
and to which she may approach very closely. 
Thirdly. Lights which have both these duties to perform, being those which are 
placed to warn a vessel from a danger at a considerable distance in one direction, 
while they may be approached with safety in another. 
In the first we require great illuminating power and a long duration of the brightest 
period, with a small angle of vertical divergence. In the second less illuminating 
power is required, but a large angle of vertical divergence, the duration of the ex- 
treme brightness not being of so much importance. In the third we require great 
