for Light-Houses and for Burning Glasses • 165 
Lenses have been long used in England* for the purposes of il- 
lumination in light-houses ; and in 1818 or 1820, some experi- 
ments had been made with them in France, in connection with 
a very powerful lamp, the particulars of which were communi- 
cated by Major Colby to Mr Robert Stevenson, Engineer to the 
Northern or Scotch Light-House Board. On the receipt of this 
letter, Mr Stevenson, ever anxious for improvements, communi- 
cated to me his intention of investigating the subject in refe- 
rence to the use of lenses in light-houses. I immediately pointed 
©ut to him the improvements in the construction of lenses, and 
the method of arranging them for the purpose of illumination, 
(which is just the converse of the arrangement for combustion), 
that I had suggested in the Edinburgh Encyclopaedia ; and he 
proposed that we should make some experiments, with the view 
of introducing them into the Northern Light-Houses, Before 
proceeding, however, to this inquiry, he was anxious to obtain 
an account of what had been done in France ; and as it was 
afterwards understood that the Corduan Light on the coast of 
France was to be fitted up with lenses, Mr Stevenson’s . intention 
was to make personal observations upon it, whenever the altera- 
tions on that light-house should be completed. 
About a fortnight ago, however, Mr Stevenson and I received 
copies of a Memoir by M. Fresnel, (already celebrated by his 
optical discoveries), read at the meeting of the Academy of 
Sciences, on the 29th July 1822, and entitled 66 Memoir e sur un 
Nouveauoc Systeme (FEclairage des Phares .” 
The method described in this Memoir, consists in using large 
lenses, built of separate pieces, in the same manner as those which 
I proposed ten years ago, and the plan of combining them with 
reflectors is also similar to that which is above described. 
In making this claim to the invention of polyzonal lenses, it 
will not be understood that I mean to throw any blame upon 
M. Fresnel, for not having made a reference to the article in 
which it is contained. I am quite satisfied that this distinguish- 
ed philosopher never saw the work in which it is published, and 
that he will be the first to acknowledge the priority of my la- 
bours. 
* Lenses twenty-two inches in diameter, are employed in the lower light-house 
in the Isle of Portland, erected in 1789. 
