i8o 



NA TURE 



{June 23, 1887 



where the light is established. The beams thus sent out 

 may be white or coloured, the differences in coloured 

 media themselves, or, as compared with white light, being 

 equalized approximately by the instruments used. The 

 condensing method has been applied more freely to the 

 smaller than to the larger orders of apparatus during the 

 past twenty-five years: and among the most beautiful 

 illustrations of the system, designed not alone by Mr. 

 Stevenson, but by Mr. Chance, Mr. Alan Brebner, and 

 Dr. Hopkinson, may be cited the Buddonness, the Isle 

 Oronsay, the Lochindaal, the Dartmouth, the Hoylake, 

 and many apparatus for certain narrow seas in Australia. 

 But the large lights of Orme's Head, Dungeness, Bidston, 

 Longships, St. Tudwal's, Dublin Bay, and McArthur's 

 Head, may also be selected as good examples of the con- 

 densing plan. 



A third and very valuable improvement is the group- 

 flashing system of Dr. John Hopkinson, F.R.S., by which 

 a new series of characteristics has been added to revolv- 

 ing lights. This invention dates from 1874, and consists 

 in so shaping and combining on uneciual axes the panels 

 of an apparatus that a double, triple, or fourfold flash 

 may be produced, each flash of the group being of such 

 duration and divided from another flash by such an 

 interval of time that compass-bearings may easily be 

 taken from the ship ; while each group is separated from 

 another group by a longer interval, the whole period being 

 one of the usual periods of revolving lights, such as half a 

 minute. Thus, while adequate power is maintained for 

 each flash, an unmistakable distinction is established. 

 This plan became rapidly popular. The Trinity House 

 were the first to apply it, in 1875, to the catoptric floating 

 light on the Royal Sovereign Shoals, near Hastings. The 

 next applications were to a dioptric light for Mexico, and 

 to the Little Basses light, Ceylon. It is now used all 

 over the world. At the Casquets, in 1876, it enabled the 

 Trinity Corporation to dispense with two of the three 

 lights hitherto employed, and show from one tower a half- 

 minute light in triple flashes, each lasting two seconds, 

 each interval between them three seconds, and the long 

 interval between the groups eighteen seconds. The great 

 lights of Bull Point, Hartland Point, and Eddystone are 

 other examples of double and triple group-flashing by 

 optical combinations. 



The use of colour in lighthouse practice has been gra- 

 dually diminishing since 1837, and is now almost re- 

 stricted to harbour-lights and ship-lights, with a few 

 cases of fixed sea-lights where a danger is to be marked 

 over a narrow sector. The loss by absorption in red and 

 green, the only two colours available, being from 60 to 

 80 per cent. — a loss slightly redeemed in the case of red 

 by a certain relative superiority to white in thick weather 

 — it is obvious that colour must sooner or later disappear 

 from the list of effective lighthouse agents. Meanwhile 

 the power of a coloured beam (without regard to the 

 illuminant) has been optically enhanced by one of two 

 methods, superficial amplitude and azimuthal condensa- 

 tion. 



Where a revolving light is to show, in alternate or 

 other series, red and white beams, the power may be 

 approximately equalized by assigning to the red a certain 

 greater angular breadth in the panels of prisms and lenses 

 than to the white. The Wolf Rock light (1869), the 

 Flamborough Head (1872), the Hartland Point (1874), 

 were so treated by Mr. James Chance, though with 

 different arrangements of panels, the average proportion 

 being 73 for the red, and 27 for the white. The 

 coloured glass plates used were of a selected tint of 

 " copper ruby." The second method, condensation, is 

 mainly applicable, as before mentioned, by means of 

 vertical prisms and other agents to lighting sectors of the 

 horizon, or to securing perfect definition between two 

 coloured arcs or between a white and a coloured arc. 

 The Kingswear fourth-order light, Dartmouth (1865), 



designed by jMr. Chance, is an excellent example. In a 

 seaward arc of 45" there is a central white beam of 9.^'^ 

 between a red beam of 175^ and a green beam of I7f'^. 

 Ten vertical prisms were used, four condensing the lights 

 on the border of the red and white, and four on the 

 border of the green and white, while two augmented the 

 central beam. The fairway channel to the harbour is 

 indicated by the coloured light, and the bright beam con- 

 stitutes a sea-light which is frequently observed at a 

 distance of sixteen miles, though the lamp is inferior to 

 the lamps of to-day. 



The signal-lights of the port and starboard sides of a 

 vessel are coloured in order that a marked contrast may 

 be visible at a distance of at least two miles, and her 

 course and evolutions plainly understood. But the 

 great inferiority of green to red, and of both to white 

 (the third signal carried by a steamer being a white light), 

 combined with the imperfection of the optical apparatus 

 and of the burner used, renders too many ship-lights 

 lamentably untrustworthy at even this short range, and 

 can only tend to multiply such terrible collisions as those 

 with which we have become familiar during the past 

 fifteen years. It might be impracticable, on account of 

 weight or cost, to introduce condensing agents into side- 

 lights generally, though Mr. Thomas Stevenson, ever 

 foremost in the van of improvement, tried them on the 

 small steamer Pharos in 1866; but there can be no 

 sufficient reason for not adopting such lenses of true 

 lighthouse types as are now made for the purpose in Bir- 

 mingham and Paris, and in not fitting them with the 

 incandescent electric light in two different degrees of power, 

 so as to equalize nearly the red and green lights, and in not 

 making them both equal in visibility to the white ; thus 

 securing an effective signal for the adequate protection of 

 life and property at sea. The writer has long, but with 

 small success, advocated this course. Public opinion, 

 however, may yet be stimulated by some crowning 

 disaster to insist on a reform so urgently needed, and 

 so perfectly easy to realize. 



In 1873 the first dioptric light established in England, 

 Start Point, received its present apparatus in substitution 

 for the old Fresnel lenses and concave mirrors. The 

 new revolving light, the design of Mr. Chance, and which 

 was repeated in 1874 at Cape Bon, Africa, and the South 

 Stack Rock, Holyhead, was composed symmetrically of 

 six sides of 60°, with the usual upper and lower prisms, 

 the central lens having nine elements in circular settings. 

 The panels are thus the widest in azimuth hitherto con- 

 structed, except some of those of Flamborough Head, 

 which subtended 695°, or the four holophotal quadrants 

 constituting the South Stack Low Light (1879), designed 

 by Dr. Hopkinson, and the only existing light of the 

 kind. By a subsidiary arrangement of totally-reflecting 

 prisms and a holophote, a fixed red beam at Start Point 

 was projected to a lower chamber in the tower, and 

 thence sent out to mark the position of certain rocks. 

 The Watling Island (Bahamas) second-order double- 

 flashing light of 1885, designed by Dr. Hopkinson, is a 

 unique specimen of holophotal circular settings, with the 

 most recent improvements. 



A remarkable variation of the usual elements of a 

 dioptric sea-light dates from 1879 or 1880. Lower prisms 

 for sea-lights had, at the suggestion of the writer in 1874, 

 been suppressed on several occasions ; and for port- 

 lights, Messrs. Chance had dispensed with all prisms, 

 and raised the lenses to a vertical angle of 80°. But 

 now it was determined to produce a first-order apparatus 

 with refractors only, extending the vertical angle to 92° 

 from 56" or 57°, the old normal height. This was at- 

 tained by Messrs. Chance by means of dense flint glass 

 in the superior and inferior limits. The power of the lenses, 

 always counting for 75 per cent, of that of the complete 

 light, was thus considerably augmented, while the cost 

 and bulk were reduced, though doubtless at the expense 



