November 21, 1895] 



NATURE 



57 



One of the most notable of recent improvements is 

 the hyper- radiant apparatus of 1330 m.m. or even greater 

 radius, proposed by Messrs. Stevenson in 1869 (Fig. 4), 

 This apparatus was designed to take advantage of the large 

 and powerful flames of burners of greater diameter than 

 five-wick ; the introduction of burners of increased size 

 having been inaugurated by Mr. Wigham in connection 

 with his gas burners, and rendered possible, on financial 

 grounds, with oil burners, by the introduction of paraffin. 

 With revolving apparatus of 920 m.m. radius, it was 

 clearly demonstrated that with burners of large diameter, 

 much of the light, being exfocal, escaped condensation, 

 and hence the hyper-radiant apparatus was designed to 

 utilise and condense the rays proceeding from the large 

 radiant. The light from one of the lenses of a hyper- 

 radiant apparatus is more intense with the same size of 

 burner in the focus than that from two lenses of 920 

 m.m. The hyper-radiant is now largely used by all 

 lighthouse authorities. In 1872, Mr. Wigham sug- 

 gested a very obvious method of increasing the power 

 of a lighthouse by superposing in the lantern two, 

 three, or four lenses, each with its own burner. Such 

 arrangements have been introduced at several lighthouses, 



recent designs by Messrs. Stevenson, of optical apparatus 

 for New Zealand, lenses equiangular throughout have 

 been introduced (Fig. 5). 



The increased and ever-increasing speed of steamships 

 has necessitated the adoption, for over-sea and prominent 

 lighthouses, of revolving, flashing, and group flashing 

 apparatus, having short periods of light and darkness. 

 This is desirable in order that the sailor may be able, in 

 a short space of time, without appeal to a watch, to tell 

 the character of the light he sees. But long before the 

 days of fast steamers, Mr. R. Stevenson recognised the 

 desirability of shortening the periods of light and dark- 

 ness, and in 1825 he introduced the "flashing" distinction 

 on the coast of Scotland, having periods of 5" light and 

 5" darkness. Subsequent experience has fully proved 

 the utility of this striking characteristic. In 1874, Dr. 

 Hopkinson proposed the important group flashing 

 characteristic, which has since been largely adopted. 



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Fig. 



Section of Equiangular Refractor. 

 Fig. 5. — New Zealand Lights. 



both in Ireland and England, but it is difficult to see 

 what the advantage is, as it is quite easy with a single 

 apparatus, properly designed, and single burner to get the 

 same power and at a less cost. Mr. Charles A. Stevenson 

 designed a form of refractor which is spherical in the 

 horizontal and vertical planes, and this form was adopted 

 in the apparatus of North Fair Isle lighthouse. The 

 spherical refractor, if it has spherical profiles, loses in 

 efficiency if carried beyond an angle subtending 20° at the 

 focus, but when this design has combined with it Mr. C. A. 

 Stevenson's equiangular prisms, as in the design for Sule 

 Skerry lighthouse, th ere is less divergence, and hence less 

 loss of light, than is the case with the Fresnel forms ; in the 

 case of the spherical lens, from the fact of its greater 

 radius, and in th6 equiangular, from the prisms being of 

 profiles of minimum divergence for exfocal light. In the 

 design for Rattray Head apparatus, the equiangular 

 prisms have been adopted to enable the lens to be carried 

 to 80° of a vertical angle, the prisms at this high angle of 

 refraction still remaining of good efficiency ; whereas 

 when lenses are carried to this high refracting angle by 

 Fresnel's prisms, the lenses become very ineffective, 

 owing to their great divergence for exfocal light. In 



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The length of the duration of a flash to be effective 

 has been lately much discussed by lighthouse engineers 

 and is a matter of great importance, for the shorter the 

 flash can be made with efficiency, the stronger it can be 

 made. Messrs. Stevenson have, in recent years, been 

 gradually reducing the length of the flashes in apparatus 

 designed by them, and in the case of the Isle of May 

 electric light, installed in 1886, they reduced the flashes 

 to half-second duration. They have not been followed 

 in this by the lighthouse engineers of other countries, as, 

 for example, in the case of St. Catherine's, lighted in 

 1888, where the flashes are 5" duration. The French 

 lighthouse engineers, however, have gone further, and 

 within the last two years have designed lights, the 

 duration of the flashes being only one-tenth second. 

 This is going to the other extreme. M. Bourdelles and 

 M. Blondel, in papers read before the International 

 Maritime Congress, 1893, give the grounds on which 

 they advocate such short flashes. They say, and say 



NO. 1360, VOL. 53] 



