194 FRAGMENTS OF SCIENCE. 
In a first-class " dioptric " apparatus the light emanates 
from a lamp with several concentric wicks, the flame of 
which, being kindled by a very active draught, attains to 
great intensity. In fixed lights the lenses refract the rays 
issuing from the lamp so as to cause them to form a lumi- 
nous sheet which grazes the sea-horizon. In revolving lights 
the lenses gather up the rays into distinct beams, resem- 
bling the spokes of a wheel, which sweep over the sea and 
strike the eye of the mariner in succession. 
It is not for clear weather that the greatest strength- 
ening of the light is intended, for here it is not needed. 
Nor is it for densely foggy weather, for here it is in- 
effectual. But it is for the intermediate stages of hazy, 
snowy, or rainy weather, in which a powerful light can 
assert itself, while a feeble one is extinguished. The 
usual first-order lamp is one of four wicks, but Mr. Doug- 
lass, the able and indefatigable engineer of the Trinity 
House, has recently raised the number of the wicks to six, 
which produce a very noble flame. To Mr. Wighain, of 
Dublin, we are indebted for the successful application of 
gas to lighthouse illumination. In some lighthouses his 
power varies from 28 jets to 108 jets, while in the light- 
house of Galley Head three burners of the largest size can 
be employed, the maximum number of jets being 324. 
These larger powers are invoked only in case of fog, the 
28-jet burner being amply sufficient for clear weather. 
The passage from the small burner to the large, and from 
the large burner to the small, is made with ease, rapidity, 
and certainty. This employment of gas is indigenous to 
Ireland, and the Board of Trade has exercised a wise 
liberality in allowing every facility to Mr. Wigham for the 
development of his invention. 
The last great agent employed in lighthouse illumination 
is electricity. It was in this Institution, beginning in 
1831, that Faraday proved the existence and illustrated 
the laws of those induced currents which in our day have 
received such astounding development. In relation to 
this subject Faraday's words have a prophetic ring. " I 
have rather," he writes in 1831, "been desirous of dis- 
covering new facts and new relations dependent on mag- 
neto-electric induction than of exalting the force of those 
already obtained, being assured that the latter would find 
their full development hereafter." The labors of Holmes, 
