Some Account oftlie Nortliern Light-liouses. 121 



1824, however, in consequence of Colonel Colby's having in- 

 formed him that the Tour de Corduan was just fitted up with 

 lenses, he visited that light-house; and in 1825, two of the 

 lenses, and the lamp employed for illuminating them, were re- 

 ceived from Paris, where they had been ordered by Mr Steven- 

 son. In 1826, Colonel Colby made arrangements for exhibiting 

 the lens and reflector, and Lieutenant Drummond's apparatus, 

 in one of the long rooms of the Tower of London. To this 

 exhibition the Master^ and Elder Brethren of the Trinity- 

 house, Sir Wilham Rae the Lord Advocate of Scotland, and 

 other members of the Northern Lights Board then attending 

 Parliament, were invited, together with a number of the mem- 

 bers of the Royal Society, including Sir William Herschel, Dr 

 Olinthus Gregory and Mr Barlow, as also Messrs Gilbert, the 

 opticians, and other artists of eminence. On this occasion a train 

 of very interesting experiments were made. From London the 

 lens was sent to Messrs Bolton and Watt of Soho, where Mr 

 Stevenson had a consultation with some of the members of that 

 scientific and highly respectable house, regarding the construc- 

 tion of lenses for light-house purposes; but it appeared that 

 Sheffield was rather the place for that manufacture. 



The French lens was considered by Sir David Brewster as of 

 an inferior quality, from its being made of greenish glass, and he 

 stated to the Light-house Board, that if a lens were prepared 

 under his direction by Messrs Gilbert of London, its effect 

 would be much more powerful in penetrating fogs than that of 

 the French lens. The Light-house Board, therefore, though 

 they considered their lights adequate for every purpose, ex- 

 cepting penetrating fogs, upon the recommendation of Sir David 

 Brewster, readily gave him a carte blanche to prepare a lens. 

 The comparative trial of this lens with the French lens, the re- 

 flected light at present in use and the lime-ball light, and the 

 ascertaining of their respective powers in penetrating fog, were 

 the primary objects of the late experiments at GuUan Hill. 



In the comparison of the French and British lenses, the first 

 of which is of greenish glass, and that prepared under the direc- 

 tions of Sir David Brewster of flint glass, there was no percep- 

 tible advantage on either side, as observed from the Calton 

 Hill, which is distant about 12 miles from Gullan. This result, 



