250 Mr Sang's Notice of a 



The harbour Commissioners, having in the summer of 1836, 

 resolved to place a light at the east pier, my brother, Mr John 

 Sang, suggested to them the propriety of surrounding the in- 

 tended gas-burner with an annular lens, so as to render useful 

 the light that otherwise would have proceeded upwards. Hav- 

 ing, however, felt some doubt as to the possibility of construc- 

 ting a lens of this kind on so small a scale, he consulted me on 

 the matter, and the ultimate result was, that I undertook to 

 supply the lens. 



My object in undertaking, at that time, such a task was two- 

 fold. In the first place, I was desirous that no difficulty either 

 in expense or in workmanship, should prevent such a benefit 

 to the harbour ; and in the second place, having been engaged 

 in a long series of experiments on the art of cutting, and having 

 arrived at what I conceive to be some general principles, I was 

 willing to regard the formation of the lens as one of those ex- 

 periments, or rather as a kind of test of the truth of the de- 

 tected laws. The entire success of the attempt, has exhibited 

 the possibility of turning or planing glass and of polishing it, to 

 almost any required figure, and that with a degree of precision 

 sufficient for many optical purposes. 



The annular lens invented by M. Fresnel, and applied by 

 him to the Phares of the French Coasts, as also by Mr Alan 

 Stevenson to some of our lighthouses, is a solid of revolution 



tions made in the process of grinding, is very simple ; and consists partly in 

 reversing the motion of the cutter, or of the chuck on which the lens is 

 placed, and partly in altering the ratio of the velocities of the surfaces in con- 

 tact. Any degree of obliquity in the direction of these scratches may in this 

 way be produced, both from right to left and from left to right, and thus every 

 possible variety in their direction must be the result ; so that the whole effect 

 ordinarily produced by ci'ossing the motions in the usual grinding process, 

 may be obtained. One would, therefore, be induced to expect great accuracy 

 from this method; and Mr Sang has certainly succeeded in giving to the 

 Kirkaldy apparatus, a very fine polish, which is a matter of great importance. 



We consider Mr Sang's labours as important in regard to the manufac- 

 ture of Lighthouse apparatus, and as calculated to improve the manufac- 

 ture of refracting instruments generally ; and we would, therefore, beg 

 leave to recommend that his communication be made known to the public. 



Robert Stevenson. 

 Alex. Adie. 

 William Galbbaith, Convr. 



Edinburgh, June 20. 1838. 



