June 23, 1887] 



NA TURK 



181 



of symmetry. The first-order lights, Anvil Point (Dorset), 

 llie Eddyslone, and the Minicoy (Indian Sea), were con- 

 structed on this principle at Birmingham (1880-83). In 

 the case of the Eddystone, two apparatus exactly alike 

 were employed by the Trinity House— one superposed 

 on the other, and each lighted by its own lamp, the 

 whole height of optical glass exceeding 12 feet. The 

 plan of superposed lenses was first suggested, in 1859, by 

 Mr. J. W. D. Brown, of Lcwisham, and first practically 

 set forth, in 1872, by Mr. John R. Wigham, an engineer 

 of conspicuous ability, in connexion with his large gas 

 flames for Irish lighthouses ; and it has been since fully 

 approved and adopted by the Trinity House. The great 

 lights of Galley Head, Howth Bailey, and Rockabill 

 attest the excellence of this arrangement of lenses, and 

 the Eddystone biform (1881) is not less successful. 



The enhancement of illuminating power through the 

 amplification, vertical and horizontal, of lenticular panels 

 has been described. But a more emphatic change, asso- 

 ciated with the name of Stevenson, has recently been 

 consummated. The radius or focal distance of Fresnel's 

 first-order light is 920 millimetres. The Fresnel of our 

 time proposed a radius of 1330, and such a lens has 

 been already constructed in France. The name "hyper- 

 radiant," given to it by Mr. Stevenson, seems hardly so 

 accurately formed as " hyper-radial," which was inde- 

 pendently suggested by the writer in 1885, although the 

 new lens will be excellently adapted to the large flames of 

 the day, at once utilizing their volume and not suffering 

 from their heat. In the lights for the Bishop Rock and 

 Round Island (Scilly) now (1887) being prepared by 

 Messrs. Chance for the Trinity House, the apparatus will 

 be of the hyper-radial type, and it will have a vertical 

 angle of 80', with glass all of the usual refractive index. 

 There will be for each lighthouse a biform structure 

 15 feet high, the Bishop having lenses for white double 

 flashes arranged in a pentagon of five groups, each lens 

 subtending 36^^ horizontally, with an eight-wick burner ; 

 and the Round Island having lenses for red single flashes, 

 each lens subtending 60' horizontally, with a ten-wick 

 burner. Petroleum will be used in both cases. The latter 

 apparatus would seem to mark the maximum limit of 

 dimension, with regard to optical agents and to illumin- 

 ants, compatible with the present conditions of lanterns 

 and towers. Hyper-radial apparatus is also being pre- 

 pared in Paris for the Tory Island and Bull Rock lights 

 in Ireland. 



But the true maximum of power or intensity for light- 

 houses must ever be sought in the electric light. This 

 application of the branch of physical science that has 

 perhaps more than any other distinguished the Victorian 

 epoch had its experimental beginnings, under the auspices 

 of Faraday, at Dungeness and the South Foreland. The 

 apparatus used at Dungeness was of 150 millimetres 

 radius. In 1881 the apparatus for Macquarie was con- 

 structed of 920 millimetres radius. Six large electric 

 lights have been established in Britain since 1862, all the 

 work of Messrs. Chance, and all of their design except 

 the Isle of May, which was planned by Mr. Thomas 

 Stevenson. The Souter Point light, revolving, of second 

 and third order elements, dates from 1871 ; the South 

 I'oreland, High and Low, fixed, of the third order, from 

 1872 ; the Lizard fixed lights, of the third order, from 

 1877 ; and the Isle of May, which gives a fourfold flash, 

 and is of first and second order radii, from 1886 In 

 addition, there have been designed by Dr. Hopkinson, 

 and made at Birmingham, the Macquarie (Sydney), a 

 first-order revolving, the most powerful light in the 

 world, and the Tino (Spezia), a second-order triple group- 

 I flashing light. It is needless to give details of these appa- 

 [ratus, which are throughout distinguished by skilful optical 

 l:ombinations and the utmost precision of workmanship. 

 They have all been, with the exception of the Isle of 

 May, the subject of elaborate papers and exhaustive dis- 



cussion before the Institution of Civil Engineers. An 

 apparatus of the second order is being prepared at 

 Birmingham for the new electric light of St. Catherine's 

 (Isle of Wight). It is composed of refractors only, 

 extended to 97' of vertical angle, and with certain special 

 arrangements for divergence. The carbons will be of 

 50 millimetres diameter and of a novel and perfect form. 



There has been during the past fifty years, but 

 especially since 1861, with regard to lighthouse charac- 

 teristics, a selective process in operation by which the 

 fittest have survived. Not only has the optical apparatus 

 been perfected in curvature, finish, and adjustment to 

 nautical conditions, and the intensity of light increased 

 threefold, but the weaker forms of distinction have been 

 suppressed, and the better forms retained and multiplied. 

 Fixed lights for the most part have been discontinued, 

 and, in this country at least, lights composed of fixed and 

 revolving portions. Long periods in revolving lights have 

 been altered to short periods, the uncertain aid of colour 

 largely abandoned, the varieties of the group-flashing 

 system invoked, and the quick contrasts of light and 

 dark resorted to in occulting or intermittent apparatus, 

 although the very ingenious but too complicated plan of 

 Babbage, with its rhythmical longs and shorts, has not 

 prevailed. The enhanced speed of steam-vessels, the 

 multiplication of all kinds of vessels, the improvement of 

 shore-lights, and the spread of commercial enterprise, by 

 which new ports are opened and new coasts explored, 

 have naturally effected these changes. And, pari passu, 

 striking improvements in the mechanism of revolving 

 carriages and of clockwork both with weights and springs, 

 in occulting-cylinders and gun-metal framing of appa- 

 ratus, have resulted from the combined eftbrts of our best 

 lighthouse engineers. 



The early rivalry between the catoptric and the dioptric 

 systems has wholly ceased, the latter having, by the 

 weight of its general and well-tried superiority, displaced 

 the old system in all directions save in one or two revolv- 

 ing sea lights of exceptional merit, like Beachy Head or 

 St. Agnes, and save in all light-vessels where the excellent 

 21-inch reflectors, with the two-wick Douglass burners, 

 often send out beams of 20,000 candles over the shoal- 

 beset waters. 



There were in the United Kingdom, in 1886, 202 sea- 

 lights, of which 147 were dioptric and 55 catoptric, and, 

 in addition, about 450 small lights of all kinds, making, 

 with the 74 light-vessels, a total of about 730. Surely 

 this is a noble growth of lighthouse illumination, even in 

 the long period under review. It compares not un- 

 favourably with the United States, the first country to 

 adopt the lenticular system on a bold and comprehensive 

 scale, or even with the country of Fresnel himself and of 

 his brother Ldonor, where the elucidations and experi- 

 ments of Allard and of Reynaud, and the practical work 

 of Lepaute, Sautter, Barbier and Fenestre, have done 

 much to promote science and benefit humanity. 



J. Kenward. 

 ( To be continued.) 



THE OBSERVATORIES AT OXFORD AND 

 CAMBRIDGE. 



THE following is the Annual Report of the Rev. Prof. 

 Pritchard, the Savilian Professor of Astronomy 

 at Oxford, to the Board of Visitors of the University 

 Observatory ; read June 8, 1887 :— 



I. Lectures. — The statutable lectures have been given, 

 and the Observatory and its instruments have been freely 

 accessible to the students during every day of Term time. 

 For next Term I ofl'er a course of elementary lectures 

 expressed as far as possible in untechnical language. I 

 desire to add also two public lectures on the development 

 of astronomy during the last century. 



