ON STANDAKUS OF LIGHT. 39 



Fourth Report of the Committee, consisting of Professor Gr. Forbes 

 (Secretary), Captain Abney, Dr. J. Hopkinson, Professor W. G. 

 Adams, Professor Gr. C. Foster, Lord Eayleigh, Mr. Preece, 

 Professor Schuster, Professor Dewar, Mr. A. Vernon Harcourt, 

 Ml-. H. Trueman Wood, Sir James Douglass, Professor H. B. 

 Dixon, and Mr. Dibdin, appointed for the purpose of reporting 

 on Standards of Light. 



The Committee, which has now been in existence for four years, has at 

 length arrived at certain definite conclusions as to the value of the dif- 

 ferent standards of light at present available. These conclusions have 

 been arrived at mainly as the result of a large number of experiments 

 undertaken by various members of the Committee. 



The experiments of the past year are added to this report in the form 

 of two appendices. The first contains an extensive series of experiments 

 which have been tabulated, showing the relative constancy of the follow- 

 ing proposed standards : — 



1. Ordinary candles made by Messrs. Millei. 



2. Ordinary candles made by Messrs. Brecknell and Turner. 



3. Spei-m candles, of six to the pound, of larger diameter, made by 



Messrs. Miller. 



4. The Pentane Standard. 



5. The Pentane Lamp. 



6. The Amyl-acetate Lamp. • 



The other appendix relates to some experiments carried on with 

 a view to making platinum heated to its melting-point a practical 

 standard. 



The Committee wishes to state that the result of all its experi- 

 ments has been to confirm the conclusions arrived at by Mr. Dibdin in 

 his report to the Metropolitan Board of Works in 1887. 



Your Committee, in making its final report, is anxious to draw atten- 

 tion to a number of conclusions which will now be treated in order :— 



(1) The present standard candle, as defined by Act of Parliament, is 

 not worthy, in the present state of science, of being called a standard, 

 and does not meet the practical requirements of those whose duty it is to 

 test illuminants. The objections to the candle as a standard are so 

 numerous, and most of them are so well understood, that space need not 

 be taken up now in repeating them. It will suffice to say that the sper- 

 maceti employed is not a definite chemical substance and is mixed with 

 other materials, and the constitution of the wick is not su.fficiently well- 

 defined, so that so-called standard candles, conforming to the definitions 

 of the Act of Parliament, can be made which vary largely in illuminating 

 power. The Committee wishes to add the important observation, which 

 has been incontestably proved by the independent observations of dif- 

 ferent members of the Committee, that the illuminating power of a 

 candle in a closed photometer, or in any small, ill-ventilated room, is 

 considerably less than in an ordinary room, 



That which forces itself upon the attention of any one who attempts 

 to determine the value of any source of light by comparison with standard 

 candles is the fluctuation fi'om minute to minute, which is due to the 

 varying length and form of the wick and the filling and emptying of the 



