ON STANDARDS OF WHITE LIGHT. 61 
Report of the Committee, consisting of Professor G. ForBes (Secre- 
tary), Captain ABNry, Dr. J. Hopkinson, Professor W. G. ADAMS, 
Professor G. C. Foster, Lord RayLeicH, Mr. PREECE, Professor 
ScuustErR, Professor DEwar, Mr. A. VERNON Harcourt, and Pro- 
fessor AYRTON, appointed for the purpose of reporting on Stand- 
ards of White Light. Drawn wp by Professor G. ForBEs. 
Tue experimental work of the Committee during the past year has not 
been extensive, as they had no funds at their disposal for experimental 
research, and they have been chiefly occupied with reviewing what has 
been done in the past and laying plans for future operations. 
Lord Rayleigh has constructed an instrument which he calls a mono- 
chromatic telescope, by means of which the illuminated screens of a photo- 
meter may be examined, allowing light only of one definite colour to pass. 
It was hoped by Lord Rayleigh that experiment might show that, with 
some suitably chosen colour, this instrument, used with any ordinary 
photometer, would, in comparing lights of different intensities and tem- 
peratures, give to each a candle-power which would be sufficiently 
accurate to represent for commercial purposes the intensity of the light. 
The Secretary has made some experiments at the Society of Arts, where 
he was kindly permitted to use the secondary batteries and glow lamps; 
but the results so far are not definite enough to justify their publication. 
Mr. Vernon Harcourt has been engaged on an inyestigation on the 
barometrical correction to his pentane standard; and on another con- 
cerning the possibility of using lamp-shades as a protection from air 
currents. His researches are communicated independently to the meeting. 
Captain Abney and General Festing have continued their observations 
on the intensity of radiations of different wave-lengths from incandescent 
carbon and platinum filaments at different temperatures, which will go far 
to assist the Committee in their work. 
Other isolated experiments have been made by members of the Com- 
mittee, 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 ‘bec Carcel’ is also liable to vari- 
ations; and with regard to the molten platinum standard of Violle, 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 em- 
ployed is not a definite chemical substance, and is mixed with other 
materials, and the constitution of the wick is not sufficiently well defined. 
Hence it is notorious that interested parties may prepare candles con- 
forming to the definitions of the Act which shall favour either the pro- 
ducer 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 proposed 
standards, and the majority feel satisfied that, for all the present com- 
