lleport on the New Dioptric Light of the Isle of May, 177 



Mr. Alan Stevenson, the assistant-engineer, explained to be, 

 that since the divergence of the reflected beam is produced 

 by placing the lamp a little out of the focus of the mirror, the 

 central pencil of reflected light is much less scattered than the 

 lateral pencils, which in an azimuth [azimuths] exactly inter- 

 mediate between the axes of two adjacent reflectors is [are] 

 very feeble indeed. The evening of the 26th was neither re- 

 markably clear, nor remarkably the reverse, and the members 

 of the Committee were satisfied that a slight increase of haze 

 would have rendered the old light wholly invisible at the 

 town of Dunbar, which appears to be placed (in the present 

 temporary position of the reflecting system) in the line of least 

 illumination. 



The number of mirrors in the old system being twenty- 

 four*, and the divergence given to the reflected light 15°, a 

 certain portion of light is thrown all round the horizon, but 

 the intensity in different directions varies (as has been said) 

 prodigiously. The divergence of 15° in a vertical plane also 

 taking place as a necessary consequence of this method of ex- 

 panding the reflected beam, by unfocusing the lamp, pro- 

 duces a useless and most injurious expenditure of light in di- 

 rections in which it can never be seen. On the other hand, 

 the dioptric system of hoops, besides the great advantage ob- 

 tained by substituting refraction for reflection (from the less 

 loss of light) rigorously fulfils the requisite geometrical con- 

 dition of spreading a plane of light, exhausting the whole 

 available light of the lamp, of precisely equal intensity in all 

 directions. This has never been attempted by any combina- 

 tion of reflectors. 



The distance of the Isle of May from Dunbar being thirteen 

 miles, the range of one mirror on the old system extending 

 through about 15°, would not differ much from a linear di- 

 stance of three miles in a direction perpendicular to a line 

 joining Dunbar and the May. The space between maximum 

 and minimum light would therefore be about a mile and a 

 half. The space walked over must have nearly included a 

 complete series of the variations of the light. Assuming the 

 average light of the mirrors at half that of the dioptric light 

 (which certainly does not seem too much), we have a supe- 

 riority of two to one in favour of the latter at a distance oi 

 thirteen miles. These are to be considered, however, but as 

 rude ocular estimations. 



• It appears that at the Isle of May there are only twenty-two reflectors 

 of a larger size, the divergence being proportionally increased in order to 

 complete the circumference. (Initialed) J. D. F. 



Third Sef'ies. Vol. 10. No. 60. March 1837. 2 A 



