July 27, 1888.] 



SCIENTIFIC NEWS. 



81 



STANDARDS 



OF LIGHT 

 NATION. 



AND ILLUMI- 



'T'HE fineness and precision of our senses in estimating 

 quantities varies exceedingly, although they may be 

 very sensitive to relative differences of magnitude or 

 quality. One would think that the sense of vision was at 

 least as well able to make a quantitative estimate as the 

 senses of taste or touch. We have not much difficulty in 

 guessing whether there are one, two, or three lumps of 

 sugar in a cup of tea, or of telling by the muscular sense 

 of weight whether a 'bus conductor has given two, three, 

 or four pence in change ; but it is very difficult to judge 

 with the same amount of accuracy, small as this may be, 

 whether a room is illuminated by two, three, or four 

 candles ; if they are placed close together, and their effect 

 is seen, while they are themselves hidden from view. 

 Indeed, it is not easy to see much difference between the 

 illumination produced by ten candles and by twenty. 

 The increase appears more like an addition of two 

 or three candles only. Those who go early to a 

 concert are often net aware that they have been sitting 

 for some time in a room provided with about one quarter 

 of the proper illumination, until the lights are turned up, 

 when they suddenly realise that they have been waiting 

 in semi-darkness. Neither the tea drinker or the habitue 

 of the 'bus would allow himself to be thus deceived 

 about sugar or pence. 



There was no demand for a unit of light until the 

 supply of gas became a subject of legislation. It was 

 then enacted that the gas burned in an argand burner at 

 the rate of 5 cubic feet per hour should yield a light 

 equal at least to that produced by 16 sperm candles 

 burning at the rate of 120 grains per hour. It is pro- 

 bable that the most unscientific legislator was conscious (if 

 he thought twice about it, though this would be asking a 

 good deal) that this was not a very accurate unit, but it 

 is also probable that the most scientific was hardly aware 

 of the numerous causes of error, the vagueness of the 

 attempted definition, and the general untrustworthy 

 character of the so-called standard. Spermaceti is not a 

 definite chemical substance. No description of the wick 

 is given, and not only can candles be prepared to suit 

 either the producer or the consumer, but it is notorious 

 that such candles have been made. But when the 

 standard candles have been obtained, it is found that no 

 matter what care is taken to avoid draughts or to pro- 

 vide uniform temperature, the illumination produced is 

 constantly varying, and if two candles be found of 

 exactly the same quality, and which burn at the same 

 rate, they will not maintain the same light for even a few 

 minutes in succession. 



The very article for which the "Parliamentary candle " 

 was to act as a standard, viz., the argand burner taking 

 5 c. ft. of gas per hour, has been put forward as a stan- 

 dard itself, it being claimed that when the flame is 

 exactly three inches high, it gives a light of 16 candles, 

 no matter what quality of gas is used. For disputes be- 

 tween the consumer and the producer of gas, or, 

 indeed, any other illuminating agent, this is doubtless 

 sufficient, but for anything of the nature of a measure- 

 ment for the purposes of any scientific determination, it 

 falls far short of a reliable standard. 



The French " bee Carcel " is an oil lamp which gives 

 a light of about 9/5 standard candles. This is subject to 

 nearly as great fluctuations as the candle while it 

 is burning; while, of course, the most elaborate specifica- 



tion of its construction would not be sufficient to ensure a 

 tolerable accordance between lamps produced by different 

 makers, to say nothing of wicks and oil. A very simple de- 

 vice invented by Mr. J. Methven gets rid of the chief varia- 

 tions of the argand burner. He merely places a screen be- 

 fore it, and provides a rectangular hole which allows the 

 light from the central part of the flame to pass, and 

 the illumination is thus made independent of the height 

 or the diameter of the flame. The quantity of light is 

 evidently proportional to the intensity of the light of the 

 flame, and this depends on the temperature of the com- 

 bustion, and on the nature of the gas, although it is quite 



Harcourt's Pentane Lamp. 



possible that the ordinary gas of commerce does not vary 

 in quality to an extent sufficient to produce any serious 

 error. A very excellent standard for most purposes, 

 when it is in the hands of an experienced operator, is the 

 pentane lamp of Mr. Vernon Harcourt. Pentane is a 

 definite chemical substance which can be obtained with 

 little difficulty, and different specimens may be relied on 

 as identical. This lamp has no wick, and gives a very 

 steady light. The original form of the lamp is only fit 

 for a laboratory, for a current of air has to be maintained 

 in an apparatus by which one volume of liquid pentane 

 becomes vaporised and mixed with 576 volumes of air, 

 forming a mixture, when vaporised, of 20 volumes 

 of air to 7 of pentane gas. 



