382 
Proceedings of Societies. 
[May, 
in 1865, Messrs. D. and T. Stevenson had recommended a third 
order apparatus for the purpose, in their Report to the Commis- 
sioners of Northern Lighthouses. A similar result was arrived 
at by the Elder Brethren of the Trinity House, in 1869, in con- 
sequence of comparative trials instituted by Prof. Tyndall, for 
testing the relative merits of a sixth order light, and a third order 
one respectively, when used with the elecftric radiant. The 
Souter Point revolving light, which was first exhibited in January, 
1871, was described. Reasons were assigned for adopting two 
optical agents — one to condense the light in the vertical plane, 
the other to produce the required horizontal compression — 
instead of attempting, even for the refraCting part of the appa- 
ratus, to effeCt the two condensations respectively by a single 
agent. Reference was made to the proposal of Mr. Thomas 
Stephenson, M. Inst. C.E., for attaining this latter desideratum ; 
as likewise to that of Mr. Brebner, M. Inst. C.E., with a similar 
objeCt. The method actually adopted was similar to that which 
had been already employed in France for the revolving light with 
the eleCtric arc. It consisted of a fixed third order light encir- 
cling 180 0 , and of a rotating oCtagonal drum of the same height 
surrounding it. Each side of this drum, comprising three panels 
in height, was composed of vertical refraCting prisms, by which 
the light, radiating in azimuth from the inner fixed apparatus, 
was compressed horizontally into a beam of 7 0 8' divergence in 
addition to that due to the diameter of the eleCtric arc. This 
was done in such a manner that every single prism had its own 
independent divergence of the same amplitude, whereby was ob- 
tained an extent of light-emitting surface of a height of 6£ feet 
and of / 12 \ inches in breadth. Stress was laid upon its being the 
characteristic feature of the beam issuing from any one of the 
sides of this glass drum, that, in passing before the eye of the 
observer at sea, its brilliancy would, from first to last, remain 
unchanged, as distinguished from the waxing and waning ap- 
pearance of the ordinary revolving light ; consequently, at what- 
everdistance the flash might be visible, the interval of its duration 
would be the same. Attention was also directed to the valuable 
suggestion made by Mr. J. N. Douglass, M. Inst. C.E., the 
Engineer to the Trinity House, that advantage should be taken 
of the landward hemisphere of the radiant light of the eleClric 
arc, to provide a beam which should be made to issue through a 
window in the tower below the main light, in order to mark cer- 
tain dangers in Sunderland Bay ; and it was stated that 54-6 per 
cent of the rearward hemisphere of light had been thus utilised. 
The two fixed lights which were inaugurated at the South Fore- 
land, in January, 1872, were described. It was explained how 
the whole of the catadioptric zones, both upper and lower, were 
in both lights made to parallelise the rays in the usual manner. 
The light, however, incident on the refraCting portion of each 
light was distributed over the sea from the horizon to within a 
