SIR WILLIAM SIEMENS, F.R.S. 311 



before them, and he was sure they could do no less than give him 

 their thanks lor having done so. 



In the discussion of the Paper 



"ON OPTICAL APPARATUS USED IN LIGHTHOUSES, 

 By JAMES T. CHANCE, M.A., Assoc. Inst. C.E., 



MR. C. W. SIEMENS * said, it had been objected that the paper 

 was not of an engineering character, but the subject was intimately 

 connected with engineering and had been received with interest 

 by the members. Mr. Chance had confined himself to the optics 

 of lighthouses, which was a large subject by itself, although many 

 would have liked to have heard about their mechanical construc- 

 tion, on which he had so much practical experience, and also on 

 the constitution of the glass, which Mr. Siemens believed was of 

 great importance to the results obtained. The description of glass 

 used in the lenses and prisms was, he ] understood, generally flint- 

 glass that was glass which had oxide of lead for its base ; but 

 this glass varied very much in quality. A small addition of lead 

 would increase its refrangibility considerably, and he knew there 

 was difficulty in getting an even mixture at the top and bottom 

 of the glass pot. He therefore thought there must be some 

 special means of obtaining uniform refractibility, or some ready 

 means of adjustment for differences in the degree of refractibility, 

 which he would ask Mr. Chance kindly to explain. One point of 

 great interest had been touched upon, which should be fully dis- 

 cussed. The Astronomer-Royal, in going from Dover to Calais, 

 observed that at a certain distance from the two Foreland lights, 

 one dioptric and the other catoptric, the two showed no essential 

 difference in intensity, though the dioptric light was far more bril- 

 liant than the other when viewed from a short distance. No 

 explanation of this observation had been offered, and he would 



* Excerpt Minutes of Proceedings of the Institution of Civil Engineers, VoL 

 XXVI. Session 1866-67, pp. 629-532. 



