498 Proceedings of the Royal Society 
optical instruments and burners submitted for trial , they may be 
held as conclusive and exhaustive. But in the experiments made 
in Edinburgh in 1869, it was pointed out to Mr Wigham, as a result 
of these experiments, that when his 52 gas jet burner (7 inches in 
diameter), was employed in combination with lenses suited for 
utilising the light from a flame of the diameter adopted originally 
by Fresnel, the effectiveness of this larger flame was to a great 
extent lost, as much of the light was ex-focal , thus escaping conden- 
sation by the Fresnel lens. It was not, however, until a compara- 
tively recent date, that the matter of larger burners again came 
before Messrs Stevenson, as a question requiring settlement in 
practical lighthouse optics, when the Northern Lighthouse Board 
resolved to increase the size of the burners in a first order light in 
the service. 
Provision had therefore to be made for utilising, as far as possible, 
the light from this increased size of burner, and in doing so it was 
decided, on the suggestion of Mr Alan Brebner, to test by actual 
trial whether Messrs Stevenson had been right in reporting that 
Fresnel’s proportion insured the best results, and should not be 
violated. 
An experimental lens in that proportion, suited for a 6-wick 
burner, was constructed and tried at the South Foreland, the focal 
distance of the lens being increased, while the holophotal system 
which I proposed in 1850, with totally reflecting prisms concentric 
with the refracting portion of the apparatus, was also adopted. 
The lens as designed has a focal distance of 1330 mm., and 
has two reflecting prisms above and below the refracting portion, 
the whole subtending an angle of 60° horizontally and of 70° verti- 
cally. 
A panel of this design renders unnecessary the use of special flint 
glass refractors, such as are employed in the New,Eddystone appa- 
ratus, and also prevents the loss of light due to the square form 
of the Fresnel lens. 
After the experiments on electricity, gas, and oil were concluded 
at the South Foreland, this lens, before being used in the Northern 
Lighthouse service, was kindly allowed to be tested by comparative 
trials. This was done by Sir James Douglas and Mr D. A Steven- 
son by comparing the depths of the shadows thrown by the different 
