Jan. 7, 1886] 



NA TURE 



'■n 



temperatures, which will go far to assist the Committee in their 

 work. 



Other isolated experiments have been made by members of 

 the Committee, which will be published in due course. 



Most of the members have examined the experiments of the 

 Trinity Board at the South Foreland. 



Existing Standards. — A consideration of existing standards 

 convinces the Committee that the standard candle, as defined 

 by Act of Parliament, is not in any sense of the word a standard. 

 The French "bee Carcel "' is also liable to variations ; and with 

 regard to the molten platinum standard of VioIIe, it seems that 

 the difficulty of applying it is so great as to render its general 

 adoption almost impossible. 



With regard to the so-called standard candle, the spermaceti 

 employed is not a definite chemical substance, and is mixed 

 with other materials, and the constitution of the wick is not 

 sufficiently well defined. Hence i! is notorious that interested 

 parties may prepare candles conforming to the definitions of the 

 Act which shall favour either the producer or consumer to a 

 serious extent. In view of these defects of the standard candle, 

 it is a matter of great importance that a standard of light should 

 be chosen which is more certain in its indications. 



The Committee have looked into the merits of different pro- 

 posed standards, and the majority feel satisfied that, for all the 

 present commercial requirements, the pentane standard of Mr. 

 Vernon Harcourt — since it has no wick and consumes a material 

 of definite chemical composition — when properly defined, is an 

 accurate and convenient standard, and gives more accurately 

 than the so-called standard candle an illumination equal to that 

 which was intended when the Act was framed. 



Yet the Committee, while desiring to impress the Board of 

 Trade and the public with these views, do not feel inclined at 

 present to recommend the adoption of any standard for universal 

 adoption until, further information on radiation having been 

 obtained from experiment, they may learn whether or not it may 

 be possible to propose an absolute standard, founded, like elec- 

 trical and other standards, on fundamental units of measurement 

 — a standard which, for these reasons, would be acceptable to 

 all civilised nations. They are, however, inclined to look upon 

 the pentane lamp as an accurate means of obtaining an illu- 

 mination to replace the so-called standard candle. 



Pi oposcd Experimental Researches. — Radiation is measured as 

 a rate of doing work, and consequently radiation might be 

 measured in watts. The illumination (or luminous effect of 

 radiation) depends partly upon the eye, and is a certain function 

 of the total radiation. This function depends upon the wave- 

 length of the radiation, or on Ihe different wave-lengths of 

 which the radiation, if it be compound, is composed. This 

 function of the radiation perceived by the eye is partly sub- 

 jective, and varies with radiations of different wave-lengths and 

 with different eyes. Thus the illumination cannot, like the 

 radiation, be expressed directly in absolute measurement. But 

 the connection between the illumination and the radiation can 

 be determined from a large number of experiments with a large 

 number of eyes, so as to get the value of the function for the 

 normal human eye. This function, however, is constant only 

 for one source of light, or, it may be, for sources of light of 

 the same temperature. It appears, then, that, in the first in- 

 stance at least, a standard should be defined as being made of 

 a definite material at a special temperature. 



The energy required to produce a certain radiation in the case 

 of a thin filament of carbon or platinum-iridium heated by the 

 passage. of an electric current can be easily measured by the 

 ordinary electric methods, and the radiation may be measured 

 by a thermopile or a bolometer, which itself can be standardised 

 by measuring the radiation from a definite surface at 100° C, 

 compared with the same at 0° C. The electric method measures 

 the absorption of energy ; the thermopile measures the total 

 radiation. These two are identical if no energy is wasted in 

 convection within the glass bulb of the lamp, by reflection and 

 absorption of the glass, and by conduction from the terminals of 

 the filament. Capt. Abney and Col. Festing have come to the 

 conclusion that there is no sensible loss from these causes. The 

 Committee propose to investigate this further. This constitutes 

 a first research. 



No research is necessaiy to prove that with a constant tem- 

 perature of a given filament the luminosity is proportional to 

 the radiation, because each of these depends only upon the 

 amount of surface of the radiating filament. It will be neces- 

 sar)', however, to examine whether with different filaments it be 



possible to maintain them at such temperatures as shall make 

 the illumination of each proportional to the radiation. This 

 will be the case if spectrum curves, giving the intensity o 

 radiation in terms of the wave-length when made out for the 

 different sources of light, are of the same form. Thus a second 

 research must be undertaken to discover whether the infinite 

 number of spectrum radiation curves, which can be obtained 

 from a carbon filament by vaiying the current, are identical in 

 form when the filament is changed, but the material remains so 

 far as possible of constant composition. 



It will be an object for a later research to determine whether, 

 when the radiation spectrum curve of any source of light has 

 been mapped, a similar curve can be found among the infinite 

 number of curves which can be obtained from a single filament. 



The next step proposed is to examine a large number! of 

 carbon or of platinum-iridium filaments, and to find 'whether the 

 radiation spectrum curve of different specimens of the same 

 material is identical when the resistance is changed in all to x 

 times the resistance at 0° C. If this law be true, a measurement 

 of the resistance of the filament would be a convenient state- 

 ment of the nature of the radiation curve. If, then, a number 

 of filaments were thus tested to give the same radiation spectrum 

 curve, their luminosities would in all cases be proportional to 

 their radiations, or (if there be no loss in convection, conduc- 

 tion, 'absorption, and reflection) proportional to the electrical 

 energies consumed. 



Thus it might be hoped to establish a standard of white light, 

 and to define it somewhat in the following manner : — A unit of 

 light is obtained from a straight carbon filament, in the direction 

 at right angles to the middle of the filament, when the resistance 

 of the filament is one-half of its resistance at o" C, and wheti it 

 consumes 10' C.G.S. units of ehctrical energy per second. 



Since Mr. Swan has taught us how to make carbon filaments 

 of constant section by passing the material of which they are 

 composed through a die, it is conceivable that another absolute 

 standard should be possible — viz., a carbon filament of circular 

 section, with a surface, say, i-iooth sq. cm., and consuming, 

 say, 10" C.G.S. units of energy per second. 



Whether such standards are possible or not depends upon the 

 experiments of the Committee. The probability of success is 

 sufficient to render these experiments desirable. 



Proposed Later Experimental Researches. — Should these hopes 

 be realised, and an absolute standard of white light thus ob- 

 tained of a character which would commend it to the civilised 

 world, it would then become an object of the Committee to find 

 the ratio of luminosity when the radiation spectrum curve of the 

 standard filainent is varied by varying the current, and con- 

 sequently the resistance of the filament. 



Thus, by a large number of subjective experiments on human 

 eyes, a multiplier would be found to express the illumination 

 from the standard lamp, with each degree of resistance of the 

 filament. 



A research, previously hinted at.'would then be undertaken — 

 viz. , to find whether the radiation spectrum curves of all sources 

 of illumination agree with one or other of the curves of the 

 standard filament. It ii not improbable that this should be the 

 case except for the high temperature of the electric arc. 



Should this be found to be true, then photometry would be 

 very accurate, and the process would be as follows : — Adjust the 

 standard filament until its radiation spectrum curve is similar to 

 that of the light to be compared. (This would probably be best 

 done by observing the wave-length of the maximum radiation, 

 or by observing equal altitudes on either side of the maximum, 

 the instruments used being a spectroscope and a line thermopile 

 or a bolometer.) The total radiation of each is then measured 

 at equal distances by the thermopile. The resistance of the 

 filament is measured, and its intensity in terms of the unit of 

 white light obtained therefrom by the previous research. The 

 luminosity of the compared source of light is then obtained 

 directly. 



The Committee desire to be reappointed, and to enable them 

 to carry out the researches indicated they ask for a grant of 30/. 



PHYSICS AT JOHNS HOPKINS^ 



THE larne and well appointed laboratories recently erected by 

 the Trustees of the Johns Hopkins University for the 

 Chemical and Biological Departments have by contrast made 

 • From Science of December 18, 1885. 



