ELECTRICITY. 



409 



hazy atmosphere ; and sailors on board steam- 

 ers have declared that in such weather Prof. 

 Holmes' light is seen 7 miles farther than any 

 of the ordinary sorts. A practical difficulty at 

 the same time growing out of the great inten- 

 sity of the light, is said to be that navigators 

 cannot readily judge of the distance of the 

 shore or point from which the light proceeds. 



Prof. Potter, of London, is led by extensive 

 photometric experiments to question the supe- 

 riority of the dioptric system over reflectors ; in 

 other words, to doubt the advantages claimed 

 for the Fresnel lenses. He finds that in passing 

 through 2 inches of clear flint glass with highly 

 polished surfaces, about of the light is still 

 lost by reflection and absorption ; while ordi- 

 nary good looking-glass reflects from -^ to f 

 of the incident light, and highly polished spec- 

 ulum metal still more. Faraday considers that 

 the dioptric apparatus absorbs not less than 50 

 percent., while pure polished silver reflects .'.'5 

 of the total incident light. Sir J. F. W. Herschel 

 has recently proposed an improved reflector, 

 which is expected to prove a great economizer 

 of light. In this, he would render available 

 Lie-big's recent discovery, of the means of pre- 

 cipitating pure silver from its solution, contain- 

 ed in a thin shell of glass : the silver, protected 

 from all agencies that would tarnish it, is said 

 to reflect .91 of the light impinging on it. It is 

 proposed to use mirrors of this sort, and in 

 forms intended to prevent the partial disper- 

 sion aud waste of light which occur with the 

 parabolic reflector ; namely, by placing a hol- 

 low hemispherical reflector above the light, and 

 a peculiar conoidal (convex) reflector below it, 

 the arrangement being such that all the light 

 reflected above the level of the source is thrown 

 back, and with that falling below the horizon, 

 also, is then thrown off from the lower reflect- 

 or in horizontal beams. With this arrangement, 

 as with the dioptric, the entire horizon can, if 

 desirable, be illuminated. Cuts of the arrange- 

 ment for the mercury light and the form of the 

 proposed new reflector accompany the article 

 from which the facts detailed in this section 

 are in good part taken. 



Tl'n/'s Electric Light, vith Mercury. In the 

 autumn of 1861, Prof. Way visited this country 

 and made several exhibitions of his electric 

 light, one of these being (by aid of a Fresnel 

 lens of the 4th order, loaned for the occasion) 

 from Fort Washington, near Washington, D. C., 

 the signals being witnessed by the President 

 and others from the '' White House," a distance 

 of 13 miles. 



This light is produced by means of a fine 

 thread of mercury kept flowing in the position 

 answering to the focal point of a suitable re- 

 flector or Fresnel lens, this mercury being at 

 the same time rendered intensely luminous by 

 the continual passage through it as a conductor 

 of a strong current of electricity. The mer- 

 cury flows from an upper reservoir, through a 

 tube with a very fine opening, into a lower 

 one. The positive and negative wires com- 



municate with the mercury in the respective 

 reservoirs; and the circuit is complete only 

 so long as, by the opening of a stopcock, 

 the mercury is allowed to flow. The current 

 is usually generated by a battery of about 45 

 Bunsen's elements. 



In the production of the recent and some- 

 what famed magnesium light,- a continual con- 

 sumption of the fine thread of the metal goes 

 on; in other words, it is rapidly burned, as a 

 taper : and the cost of the metal is one of the 

 objections to the method. In Way's light, 

 however, the mercury is not consumed, but can 

 be used repeatedly, and without apparent dete- 

 rioration or loss. True, the electricity dashes 

 the metal to some extent against the glass tube 

 which includes the filament of mercury, thus 

 in time coating this tube with patches of the 

 metal that interfere with the transmission of 

 the light; this effect, however, is but slowly 

 produced, and the tubes are readily changed and 

 cleansed. 



Two other, and. it would appear, yet more 

 objectionable features of the mercury light, re- 

 quire mention. In the first place, this metal, 

 like most or all others (see SPECTEVM OBSEP.VA- 

 TIOXS), does not when rendered incandescent 

 shine, as do the ignited carbon points, with 

 light of all the colors found in the sunbeam. 

 In fact, it gives out rays of a limited and very 

 small number only of the sorts going to make 

 up the entire spectrum ; since, on prismatic 

 analysis, it is said to yield six r.arrow and defi- 

 nite bands of color only : viz., a brick red. a 

 yellowish orange, two emerald greens near to- 

 gether, a rich blue, and a violet ; it is in addi- 

 tion, however, very rich in actinic rays, or 

 those effective in producing the photographic 

 impression. Employed to illuminate dwellings, 

 halls, or natural scenery by night, this light 

 would accordingly show as black or gray all 

 objects having colors other than those of the 

 rays composing it. In the second place, though 

 the mercury used in producing the light is not 

 oxidized, yet the intense heat arising in the 

 filament which is rendered luminous also serves 

 to volatilize the mercury at certain points, so 

 that minute breaks in the thread of the metal 

 occur, and the light is hence a flickering, and 

 not a steady one a difficulty for which per- 

 haps no remedy can be found. 



Improvement in Holmcis 1 Magneto-Electric 

 Light. Prof. F. H. Holmes has a letter in the 

 'Athenseum" of Jan. 3. 1863, in which he states 

 that such improvements have lately been made 

 in the lamp employed in his system, that the 

 movement of the carbon points is no longer 

 effected through a delicate and complicated 

 clockwork, but by means of a single wheel 

 and pinion. The lamps in use at Dungeness 

 and at Xorthfleet. the former 7 months, the lat- 

 ter still longer, have not in all that time been 

 opened, even to be oiled. In the new arrange- 

 ment, if the light be arbitrarily extinguished, 

 it immediately relights itself, and it is no longer 

 liable to spontaneous extinction. The only 



